[media] docs-rst: Add compressed video formats used on MT8173 codec driver

Add V4L2_PIX_FMT_MT21C documentation

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>