Start implementing a proper v4l2 framework as discussed during the
Linux Plumbers Conference 2008.

Introduces v4l2_device (for device instances) and v4l2_subdev (representing
sub-device instances).
Signed-off-by: NHans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@skynet.be>
Reviewed-by: NGuennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: NAndy Walls <awalls@radix.net>
Reviewed-by: NDavid Brownell <david-b@pacbell.net>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>