V4L/DVB: v4l: add V4L2_PIX_FMT_Y4 and V4L2_PIX_FMT_Y6 pixelformats

Old 4 and 6 bit greyscale pixel formats for the old bw-qcam webcam.
This is needed to convert it to V4L2.

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