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>

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v6:
 - Clean up description and remove redundant text regarding optional
   nodes

v6.1:
 - Fix commit title
1 file changed