doc: media: Document MM21 tiled format

MM21 has been documented through two software implementation [0] in libyuv
and [2] GStreamer. This format is similar to other tiled format, though it
uses uneven tile sizes. The luma tiles are 16x32 while the chroma tile have
a subsampled size of 16x16. This is the uncompressed cousin of
V4L2_PIX_FMT_MT21C and shares its tiling pattern and alignment.

[0] https://chromium.googlesource.com/libyuv/libyuv/+/b4ddbaf549a1bf5572bf703fd2862d1eb7380c6a
[1] https://gitlab.freedesktop.org/gstreamer/gstreamer/-/commit/c9b127dae3af4b3fe78468e697cb491902425254



Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>