media: videobuf2: Add VIDEOBUF2_V4L2 Kconfig option for VB2 V4L2 part

Videobuf2 is now separate from V4L2 and can be now built without it, at
least in principle --- enabling videobuf2 in kernel configuration attempts
to compile videobuf2-v4l2.c but that will fail if CONFIG_VIDEO_V4L2 isn't
enabled.

Solve this by adding a separate Kconfig option for videobuf2-v4l2 and make
it a separate module as well. This means that drivers now need to choose
both the appropriate videobuf2 memory type
(VIDEOBUF2_{VMALLOC,DMA_CONTIG,DMA_SG}) and VIDEOBUF2_V4L2 if they need
both.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>