[media] v4l2-core: deprecate V4L2_BUF_TYPE_PRIVATE

This buffer type isn't used at all, and since it is effectively undefined
what it should do it is deprecated. The define still exists, but any
internal support for such buffers is removed.
The decisions to deprecate this was taken during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>