Add detection of source pad number for drivers aware of the media controller
API, so that rcar-vin can create device nodes to support modern drivers such
as adv7604.c (for HDMI on Lager) and the converted adv7180.c (for composite)
underneath.

Building rcar_vin gains a dependency on CONFIG_MEDIA_CONTROLLER, in
line with requirements for building the drivers associated with it.
Signed-off-by: William Towle <william.towle@codethink.co.uk>
Signed-off-by: Rob Taylor <rob.taylor@codethink.co.uk>
[ulrich.hecht+renesas@gmail.com: adapted to rcar-vin rewrite]
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>