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

Document the new compatible for stub clock driver which is used for CPU
and DDR's dynamic frequency scaling.

Signed-off-by: Leo Yan <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/hi6220-clock.txt

···
 	- "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