media: add Mediatek's MM21 format

Add Mediatek's non-compressed 8 bit block video mode. This format is
produced by the MT8183 codec and can be converted to a non-proprietary
format by the MDP3 component.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>