[media] uvcvideo: Add M420 format support

The M420 format is used by the Microsoft LifeCam Studio HD.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[laurent.pinchart@ideasonboard.com: split into v4l/uvcvideo patches]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>