staging: vc-sm-cma: Add in userspace allocation API

Replacing the functionality from the older vc-sm driver,
add in a userspace API that allows allocation of buffers,
and importing of dma-bufs.
The driver hands out dma-buf fds, therefore much of the
handling around lifespan and odd mmaps from the old driver
goes away.

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