SiFive FU540 PRCI bindings

On the FU540 family of SoCs, most system-wide clock and reset integration
is via the PRCI IP block.

Required properties:
- compatible: Should be "sifive,<chip>-prci".  Only one value is
	supported: "sifive,fu540-c000-prci"
- reg: Should describe the PRCI's register target physical address region
- clocks: Should point to the hfclk device tree node and the rtcclk
          device tree node.  The RTC clock here is not a time-of-day clock,
	  but is instead a high-stability clock source for system timers
	  and cycle counters.
- #clock-cells: Should be <1>

The clock consumer should specify the desired clock via the clock ID
macros defined in include/dt-bindings/clock/sifive-fu540-prci.h.
These macros begin with PRCI_CLK_.

The hfclk and rtcclk nodes are required, and represent physical
crystals or resonators located on the PCB.  These nodes should be present
underneath /, rather than /soc.

Examples:

/* under /, in PCB-specific DT data */
hfclk: hfclk {
	#clock-cells = <0>;
	compatible = "fixed-clock";
	clock-frequency = <33333333>;
	clock-output-names = "hfclk";
};
rtcclk: rtcclk {
	#clock-cells = <0>;
	compatible = "fixed-clock";
	clock-frequency = <1000000>;
	clock-output-names = "rtcclk";
};

/* under /soc, in SoC-specific DT data */
prci: clock-controller@10000000 {
	compatible = "sifive,fu540-c000-prci";
	reg = <0x0 0x10000000 0x0 0x1000>;
	clocks = <&hfclk>, <&rtcclk>;
	#clock-cells = <1>;
};