V4L/DVB (9897): v4l2: Add camera zoom controls

The zoom controls move the zoom lens group to a an absolute position, as a
relative displacement or at a given speed until reaching physical device
limits. Positive values move the zoom lens group towards the telephoto
direction, negative values towards the wide-angle direction.

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