media: videodev.h: add PIX_FMT_FWHT for use with vicodec

Add a new pixelformat for the vicodec software codec using the
Fast Walsh Hadamard Transform.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>