arm64: hi6220: Document devicetree bindings for Hisilicon hi6220 SoC

This patch adds documentation for the devicetree bindings used by the
DT files of Hisilicon hi6220 SoC mobile platform.

Signed-off-by: Bintian Wang <bintian.wang@huawei.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>