media: mtk-vcodec: replace GPLv2 with SPDX

Replace the GPLv2 boilerplate with the corresponding SPDX reference.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>