The V4L2_PIX_FMT_HM12 format is actually a simple NV12 tiled format,
with 16x16 linear tiles. Rename the format and move its documentation
together with the other tiled NV12 formats.

Keep V4L2_PIX_FMT_HM12 for application compatibility.
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>