[media] v4l: Add a class and a set of controls for flash devices

Add a control class and a set of controls to support LED and Xenon flash
devices. An example of such a device is the adp1653.

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>