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>

diff --git a/Documentation/devicetree/bindings/clock/hi6220-clock.txt b/Documentation/devicetree/bindings/clock/hi6220-clock.txt
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+* 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>.