[am335x] Fallback to standard video interface bindings when using Linux dts

Historically AM335x LCDC driver used non-standard "hdmi" property to
refer to HDMI framer. There is no such thing in upstream DTS, so to
handle both cases fallback to bindings described in
bindings/media/video-interfaces.txt in Linux documentation.

We still make some assumptions that are not universally true: we
assume that if remote endpoint is available it's going to be HDMI
framer. Which is true for AM335x-based devices currently supported
but may be not true for some custom hardware.

MFC after: 1 week