[media] DocBook/device-drivers: Add drivers/media core stuff

There are lots of docbook marks at the media subsystem, but
those aren't used.

Add the core headers/code in order to start generating docs.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>