V4L/DVB: V4L2: mediabus: add 12-bit Bayer and YUV420 pixel formats

These formats belong to the standard format set, defined by the MIPI CSI-2
specification.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>