media: rcar-vin: move subdevice handling to async callbacks

In preparation for Gen3 support move the subdevice initialization and
clean up from rvin_v4l2_{register,unregister}() directly to the async
callbacks. This simplifies the addition of Gen3 support as the
rvin_v4l2_register() can be shared for both Gen2 and Gen3 while direct
subdevice control are only used on Gen2.

While moving this code drop a large comment which is copied from the
framework documentation and fold rvin_mbus_supported() into its only
caller. Also move the initialization and cleanup code to separate
functions to increase readability.

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>