media: rcar-vin: Generate a VIN group ID for Gen2

Prepare to move Gen2 and earlier models to media controller by
generating a unique VIN group id for each VIN instance. On Gen3 and Gen4
it is important to have a specific id in the group as media graph routes
depend on this. On Gen2 and earlier models all that will matter is to
have a unique id in the range.

Break out the id generation to a own function keeping the logic for Gen3
and Gen4 while generating a sequential id for Gen2 models.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20250613153434.2001800-6-niklas.soderlund+renesas@ragnatech.se


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>