media: Clean V4L2_PIX_FMT_NV12MT documentation

Add more information about V4L2_PIX_FMT_NV12MT and
V4L2_PIX_FMT_NV12M_16X16, so it's clearer for driver authors and users.

Also, group the two pixel formats with the other tiled formats,
for clarity.

Unlike the recently introduced tiled formats (V4L2_PIX_FMT_NV12_4L4, etc)
these formats have remained Samsung-specific until now. Therefore, and
although the NV12MT and NV12MT_16X16 nomenclatures are less clear, we are
keeping them as-is.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>