media: rcar-vin: change name of video device

The rcar-vin driver needs to be part of a media controller to support
Gen3. Give each VIN instance a unique name so it can be referenced from
userspace.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
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>