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>