media: v4l2-core: v4l2-mc: Add SPDX license identifier

Replace GPL license statement with SPDX GPL-2.0 license identifier.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>