V4L/DVB (6108): videodev2.h: add new pixel formats for the cx23415 OSD

The Conexant cx23415 MPEG encoder/decoder supports some unusual pixelformats
for the On-Screen Display. Add new defines to videodev2.h for these formats.

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