clk: hi6220: Document devicetree bindings for hi6220 clock

* Hisilicon Hi6220 Clock Controller

Clock control registers reside in different Hi6220 system controllers,

please refer the following document to know more about the binding rules

for these system controllers:

Documentation/devicetree/bindings/arm/hisilicon/hisilicon.txt

Required Properties:

- compatible: the compatible should be one of the following strings to

	indicate the clock controller functionality.

	- "hisilicon,hi6220-aoctrl"

	- "hisilicon,hi6220-sysctrl"

	- "hisilicon,hi6220-mediactrl"

	- "hisilicon,hi6220-pmctrl"

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

  region.

- #clock-cells: should be 1.

For example:

	sys_ctrl: sys_ctrl@f7030000 {

		compatible = "hisilicon,hi6220-sysctrl", "syscon";

		reg = <0x0 0xf7030000 0x0 0x2000>;

		#clock-cells = <1>;

	};

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

to specify the clock which they consume.

All these identifier could be found in <dt-bindings/clock/hi6220-clock.h>.