[media] v4l: v4l2_subdev pad-level operations

Add a v4l2_subdev_pad_ops structure for the operations that need to be
performed at the pad level such as format-related operations.

Pad format-related operations use v4l2_mbus_framefmt instead of
v4l2_format.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>