[media] v4l: Add subdev selections documentation

Add documentation for V4L2 subdev selection API. This changes also
experimental V4L2 subdev API so that scaling now works through selection API
only.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>