[media] v4l: vsp1: Make the BRU optional

Not all VSP instances have a BRU on R-Car Gen3, make it optional. Set
the feature unconditionally for now, this will be fixed when adding Gen3
support.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>