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

Documentation/devicetree/bindings/display/rockchip/dw_hdmi-rockchip.txt

@@ -34,6 +34,8 @@ Optional properties
 - clock-names: May contain "cec" as defined in dw_hdmi.txt.
 - clock-names: May contain "grf", power for grf io.
 - clock-names: May contain "vpll", external clock for some hdmi phy.
+- phys: from general PHY binding: the phandle for the PHY device.
+- phy-names: Should be "hdmi" if phys references an external phy.
 
 Example: