media: rcar-vin: add Gen3 devicetree bindings documentation

Document the devicetree bindings for the CSI-2 inputs available on Gen3.

There is a need to add a custom property 'renesas,id' and to define
which CSI-2 input is described in which endpoint under the port@1 node.
This information is needed since there are a set of predefined routes
between each VIN and CSI-2 block. This routing table will be kept
inside the driver but in order for it to act on it it must know which
VIN and CSI-2 is which.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>