media: rcar-vin: add group allocator functions

In media controller mode all VIN instances needs to be part of the same
media graph. There is also a need for each VIN instance to know about
and in some cases be able to communicate with other VIN instances.

Add an allocator framework where the first VIN instance to be probed
creates a shared data structure and registers a media device.
Consecutive VINs insert themself into the global group.

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>