staging: vc04_services: Split vchiq-mmal into a module

In preparation for adding a video codec V4L2 module which also
wants to use vchiq-mmal functions, split it out into an
independent module.
The minimum number of changes have been made to achieve this
(eg straight moves where possible) so existing checkpatch
errors will still be present.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>