[media] pixfmt-reserved.rst: Improve MT21C documentation

Improve the MT21C documentation, making it clearer that this format
requires the MDP for further processing.

Also fix the fourcc (it was a fivecc :-) )

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