dt-bindings: clk: Hi6220: Document stub clock driver

	- "hisilicon,hi6220-sysctrl"

	- "hisilicon,hi6220-mediactrl"

	- "hisilicon,hi6220-pmctrl"

	- "hisilicon,hi6220-stub-clk"

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

  region.

- #clock-cells: should be 1.

For example:

Optional Properties:

- hisilicon,hi6220-clk-sram: phandle to the syscon managing the SoC internal sram;

  the driver need use the sram to pass parameters for frequency change.

- mboxes: use the label reference for the mailbox as the first parameter, the

  second parameter is the channel number.

Example 1:

	sys_ctrl: sys_ctrl@f7030000 {

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

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

		#clock-cells = <1>;

	};

Example 2:

	stub_clock: stub_clock {

		compatible = "hisilicon,hi6220-stub-clk";

		hisilicon,hi6220-clk-sram = <&sram>;

		#clock-cells = <1>;

		mboxes = <&mailbox 1>;

	};

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

to specify the clock which they consume.