Documentation: Document Hisilicon hix5hd2 sata PHY

Add necessary binding documentation SATA PHY on Hisilicon hix5hd2 soc.

Signed-off-by: Jiancheng Xue <xuejiancheng@huawei.com>
Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>