media: rcar-vin: Move group async notifier

The VIN group notifier code is intertwined with the media graph layout
code for R-Car CSI-2 subdevices, this makes it hard to extend the group
to also support the R-Car ISP channel selector.

Before breaking the two concepts apart and extending it move the group
code to its final location. There is no functional change and all
functions are moved verbatim.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>