[media] v4l: Add M420 format definition

M420 is a hybrid YUV 4:2:0 packet/planar format. Two Y lines are
followed by an interleaved U/V line.

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