Add V4L2_PIX_FMT_MT21C documentation
Signed-off-by: NTiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>