[media] uvcvideo: Support 10bit, 12bit and alternate 8bit greyscale formats

The SPCAM SP620U camera uses the alternate "Y8  " GUID for the 8-bit
greyscale format, and supports a 12-bit greyscale format. Add support
for those in the driver, as well as for 10-bit greyscale formats.

Signed-off-by: Stefan Muenzel <stefanmuenzel@googlemail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>