V4L/DVB: v4l2: Add illuminator controls

Some media devices (microscopes) may have one or many illuminators.
This patch makes them controlable by the applications.

Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>