[media] v4l: Add missing 12 bits bayer media bus formats

Add codes and documentation for the following media bus formats:

- V4L2_MBUS_FMT_SGBRG12_1X12
- V4L2_MBUS_FMT_SGRBG12_1X12
- V4L2_MBUS_FMT_SRGGB12_1X12

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>