Ingenic SoC CGU binding

The CGU in an Ingenic SoC provides all the clocks generated on-chip. It
typically includes a variety of PLLs, multiplexers, dividers & gates in order
to provide many different clock signals derived from only 2 external source
clocks.

Required properties:
- compatible : Should be "ingenic,<soctype>-cgu".
  For example "ingenic,jz4740-cgu" or "ingenic,jz4780-cgu".
- reg : The address & length of the CGU registers.
- clocks : List of phandle & clock specifiers for clocks external to the CGU.
  Two such external clocks should be specified - first the external crystal
  "ext" and second the RTC clock source "rtc".
- clock-names : List of name strings for the external clocks.
- #clock-cells: Should be 1.
  Clock consumers specify this argument to identify a clock. The valid values
  may be found in <dt-bindings/clock/<soctype>-cgu.h>.

Example SoC include file:

/ {
	cgu: jz4740-cgu {
		compatible = "ingenic,jz4740-cgu";
		reg = <0x10000000 0x100>;
		#clock-cells = <1>;
	};

	uart0: serial@10030000 {
		clocks = <&cgu JZ4740_CLK_UART0>;
	};
};

Example board file:

/ {
	ext: clock@0 {
		compatible = "fixed-clock";
		#clock-cells = <0>;
		clock-frequency = <12000000>;
	};

	rtc: clock@1 {
		compatible = "fixed-clock";
		#clock-cells = <0>;
		clock-frequency = <32768>;
	};

	&cgu {
		clocks = <&ext> <&rtc>;
		clock-names: "ext", "rtc";
	};
};