clk: gxbb: Document bindings for the GXBB clock controller

* Amlogic GXBB Clock and Reset Unit

The Amlogic GXBB clock controller generates and supplies clock to various

controllers within the SoC.

Required Properties:

- compatible: should be "amlogic,gxbb-clkc"

- reg: physical base address of the clock controller and length of memory

       mapped region.

- #clock-cells: should be 1.

Each clock is assigned an identifier and client nodes can use this identifier

to specify the clock which they consume. All available clocks are defined as

preprocessor macros in the dt-bindings/clock/gxbb-clkc.h header and can be

used in device tree sources.

Example: Clock controller node:

	clkc: clock-controller@c883c000 {

		#clock-cells = <1>;

		compatible = "amlogic,gxbb-clkc";

		reg = <0x0 0xc883c000 0x0 0x3db>;

	};

Example: UART controller node that consumes the clock generated by the clock

  controller:

	uart_AO: serial@c81004c0 {

		compatible = "amlogic,meson-uart";

		reg = <0xc81004c0 0x14>;

		interrupts = <0 90 1>;

		clocks = <&clkc CLKID_CLK81>;

		status = "disabled";

	};