V4L/DVB (9820): v4l2: add v4l2_device and v4l2_subdev structs to the v4l2 framework.

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: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Reviewed-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Andy Walls <awalls@radix.net>
Reviewed-by: David Brownell <david-b@pacbell.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>