V4L/DVB (4221): Add HM12 YUV format define.

HM12 is a YUV 4:1:1 format used by the cx2341x MPEG encoder/decoder for
the raw YUV input/output. The Y and UV planes are broken up in 16x16
macroblocks and each macroblock is transmitted in turn (row by row).

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>