media: dt-bindings: media: adv748x: Document re-mappable addresses

The ADV748x supports configurable slave addresses for its I2C pages.
Document the page names, and provide an example for setting each of the
pages explicitly.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>