media: rcar-vin: Create a group notifier

As CSI-2 subdevices are shared between several VIN instances, a shared
notifier to collect the CSI-2 async subdevices is required. So far, the
rcar-vin driver used the notifier of the last VIN instance to probe but
with the forth-coming introduction of parallel input subdevices support
in mc-compliant code path, each VIN may register its own notifier if any
parallel subdevice is connected there.

To avoid registering a notifier twice (once for parallel subdev and one
for the CSI-2 subdevs) create a group notifier, shared by all the VIN
instances.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>