Binding for Silicon Labs Si5341 and Si5340 programmable i2c clock generator.

Reference
[1] Si5341 Data Sheet
    https://www.silabs.com/documents/public/data-sheets/Si5341-40-D-DataSheet.pdf
[2] Si5341 Reference Manual
    https://www.silabs.com/documents/public/reference-manuals/Si5341-40-D-RM.pdf

The Si5341 and Si5340 are programmable i2c clock generators with up to 10 output
clocks. The chip contains a PLL that sources 5 (or 4) multisynth clocks, which
in turn can be directed to any of the 10 (or 4) outputs through a divider.
The internal structure of the clock generators can be found in [2].

The driver can be used in "as is" mode, reading the current settings from the
chip at boot, in case you have a (pre-)programmed device. If the PLL is not
configured when the driver probes, it assumes the driver must fully initialize
it.

The device type, speed grade and revision are determined runtime by probing.

The driver currently only supports XTAL input mode, and does not support any
fancy input configurations. They can still be programmed into the chip and
the driver will leave them "as is".

==I2C device node==

Required properties:
- compatible: shall be one of the following:
	"silabs,si5340" - Si5340 A/B/C/D
	"silabs,si5341" - Si5341 A/B/C/D
- reg: i2c device address, usually 0x74
- #clock-cells: from common clock binding; shall be set to 2.
	The first value is "0" for outputs, "1" for synthesizers.
	The second value is the output or synthesizer index.
- clocks: from common clock binding; list of parent clock  handles,
	corresponding to inputs. Use a fixed clock for the "xtal" input.
	At least one must be present.
- clock-names: One of: "xtal", "in0", "in1", "in2"
- vdd-supply: Regulator node for VDD

Optional properties:
- vdda-supply: Regulator node for VDDA
- vdds-supply: Regulator node for VDDS
- silabs,pll-m-num, silabs,pll-m-den: Numerator and denominator for PLL
  feedback divider. Must be such that the PLL output is in the valid range. For
  example, to create 14GHz from a 48MHz xtal, use m-num=14000 and m-den=48. Only
  the fraction matters, using 3500 and 12 will deliver the exact same result.
  If these are not specified, and the PLL is not yet programmed when the driver
  probes, the PLL will be set to 14GHz.
- silabs,reprogram: When present, the driver will always assume the device must
  be initialized, and always performs the soft-reset routine. Since this will
  temporarily stop all output clocks, don't do this if the chip is generating
  the CPU clock for example.
- interrupts: Interrupt for INTRb pin.
- #address-cells: shall be set to 1.
- #size-cells: shall be set to 0.


== Child nodes: Outputs ==

The child nodes list the output clocks.

Each of the clock outputs can be overwritten individually by using a child node.
If a child node for a clock output is not set, the configuration remains
unchanged.

Required child node properties:
- reg: number of clock output.

Optional child node properties:
- vdd-supply: Regulator node for VDD for this output. The driver selects default
	values for common-mode and amplitude based on the voltage.
- silabs,format: Output format, one of:
	1 = differential (defaults to LVDS levels)
	2 = low-power (defaults to HCSL levels)
	4 = LVCMOS
- silabs,common-mode: Manually override output common mode, see [2] for values
- silabs,amplitude: Manually override output amplitude, see [2] for values
- silabs,synth-master: boolean. If present, this output is allowed to change the
	multisynth frequency dynamically.
- silabs,silabs,disable-high: boolean. If set, the clock output is driven HIGH
	when disabled, otherwise it's driven LOW.

==Example==

/* 48MHz reference crystal */
ref48: ref48M {
	compatible = "fixed-clock";
	#clock-cells = <0>;
	clock-frequency = <48000000>;
};

i2c-master-node {
	/* Programmable clock (for logic) */
	si5341: clock-generator@74 {
		reg = <0x74>;
		compatible = "silabs,si5341";
		#clock-cells = <2>;
		#address-cells = <1>;
		#size-cells = <0>;
		clocks = <&ref48>;
		clock-names = "xtal";

		silabs,pll-m-num = <14000>; /* PLL at 14.0 GHz */
		silabs,pll-m-den = <48>;
		silabs,reprogram; /* Chips are not programmed, always reset */

		out@0 {
			reg = <0>;
			silabs,format = <1>; /* LVDS 3v3 */
			silabs,common-mode = <3>;
			silabs,amplitude = <3>;
			silabs,synth-master;
		};

		/*
		 * Output 6 configuration:
		 *  LVDS 1v8
		 */
		out@6 {
			reg = <6>;
			silabs,format = <1>; /* LVDS 1v8 */
			silabs,common-mode = <13>;
			silabs,amplitude = <3>;
		};

		/*
		 * Output 8 configuration:
		 *  HCSL 3v3
		 */
		out@8 {
			reg = <8>;
			silabs,format = <2>;
			silabs,common-mode = <11>;
			silabs,amplitude = <3>;
		};
	};
};

some-video-node {
	/* Standard clock bindings */
	clock-names = "pixel";
	clocks = <&si5341 0 7>; /* Output 7 */

	/* Set output 7 to use syntesizer 3 as its parent */
	assigned-clocks = <&si5341 0 7>, <&si5341 1 3>;
	assigned-clock-parents = <&si5341 1 3>;
	/* Set output 7 to 148.5 MHz using a synth frequency of 594 MHz */
	assigned-clock-rates = <148500000>, <594000000>;
};

some-audio-node {
	clock-names = "i2s-clk";
	clocks = <&si5341 0 0>;
	/*
	 * since output 0 is a synth-master, the synth will be automatically set
	 * to an appropriate frequency when the audio driver requests another
	 * frequency. We give control over synth 2 to this output here.
	 */
	assigned-clocks = <&si5341 0 0>;
	assigned-clock-parents = <&si5341 1 2>;
};