media: dt-bindings: media: rcar-vin: Align Gen2 and Gen3

Align description of the Gen2 and Gen3 bindings for parallel input.
This commit prepares for description of optional endpoint properties in ports
subnodes accepting parallel video connections.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>