[media] media: Media device node support

The media_devnode structure provides support for registering and
unregistering character devices using a dynamic major number. Reference
counting is handled internally, making device drivers easier to write
without having to solve the open/disconnect race condition issue over
and over again.

The code is based on video/v4l2-dev.c.

[mchehab@redhat.com: Remove linux/smp_lock.h include to not break compilation on bisect]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>