dt-bindings: add bindings doc for hi3798cv200 combphy

It adds the device tree bindings for PCIE/SATA/USB3 combo PHY found on
HiSilicon STB SoCs.

Signed-off-by: Jianguo Sun <sunjianguo1@huawei.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>