video: Add ADV751[13] DT bindings documentation

The ADV7511, ADV7511W and ADV7513 are HDMI audio and video transmitters
compatible with HDMI 1.4 and DVI 1.0. They're described in DT using the
OF graph bindings and a list of custom properties pertaining to the
input video bus configuration.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>