[media] v4l: add V4L2_PIX_FMT_Y12 format

Y12 is a grey-scale format with a depth of 12 bits per pixel stored in
16-bit words.

Signed-off-by: Michael Jones <michael.jones@matrix-vision.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>