media: platform: rename mtk-vcodec/ to mediatek/mtk-vcodec/

As the end goal is to have platform drivers split by vendor,
rename mtk-vcodec/ to mediatek/mtk-vcodec/.

Acked-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>