V4L/DVB: v4l: Add V4L2_CID_IRIS_ABSOLUTE and V4L2_CID_IRIS_RELATIVE controls

Those control, as their names imply, control the camera aperture
settings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>