clk: hi6220: Document devicetree bindings for hi6220 clock

Document DT files bindings for Hisilicon hi6220 clock.
Signed-off-by: Bintian Wang <bintian.wang@huawei.com>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Michael Turquette <mturquette@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>

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

+* 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>.