media: rcar-vin: prepare for media controller mode initialization

Prepare for media controller by calling a different initialization then
when running in device centric mode. Add trivial configuration of
the mbus and creation of the media pad for the video device entity.

While we are at it clearly mark the digital device centric notifier
functions with a comment.

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>