dt-bindings: allow optional phys in Rockchip dw_hdmi binding

Some newer Rockchip SoCs use an Innosilicon hdmiphy accessed via general
mmio, so allow these to be referenced via the regular phy interfaces
and therefore add optional phy-related properties to the binding.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180912124740.20343-4-heiko@sntech.de