dt-bindings: add bindings doc for HiSilicon INNO USB2 PHY

It adds device tree bindings document for HiSilicon INNO USB2 PHY.

Signed-off-by: Pengcheng Li <lpc.li@hisilicon.com>
Signed-off-by: Jiancheng Xue <xuejiancheng@hisilicon.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>