[media] adv7604: Add support for asynchronous probing

Register the device with the v4l2 asynchronous probing framework to
support asynchronous instantiation systems such as DT.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>