[media] v4l: add Mediatek compressed video block format

Add V4L2_PIX_FMT_MT21C format used on MT8173 driver.
It is compressed format and need MT8173 MDP driver to transfer to other
standard format.

Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>