media: adv748x: Add adv7481, adv7482 bindings

Create device tree bindings documentation for the ADV748x.
The ADV748x supports both the ADV7481 and ADV7482 chips which
provide analogue decoding and HDMI receiving capabilities

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>