dt-bindings: add binding for Rockchip hdmi phy using an Innosilicon IP

The phy is used so far in two Rockchip socs the rk3228 and the rk3328.

Signed-off-by: Zheng Yang <zhengyang@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>