[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>

Documentation/media/uapi/v4l/pixfmt-reserved.rst

@@ -237,12 +237,12 @@ please make a proposal on the linux-media mailing list.
     * .. _V4L2-PIX-FMT-MT21C:
 
       - ``V4L2_PIX_FMT_MT21C``
-      - 'MT21C'
+      - 'MT21'
       - Compressed two-planar YVU420 format used by Mediatek MT8173.
 	The compression is lossless.
-        It is an opaque intermediate format, and MDP HW could convert
-        V4L2_PIX_FMT_MT21C to V4L2_PIX_FMT_NV12M,
-        V4L2_PIX_FMT_YUV420M and V4L2_PIX_FMT_YVU420.
+	It is an opaque intermediate format and the MDP hardware must be
+	used to convert ``V4L2_PIX_FMT_MT21C`` to ``V4L2_PIX_FMT_NV12M``,
+	``V4L2_PIX_FMT_YUV420M`` or ``V4L2_PIX_FMT_YVU420``.
 
 
 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|