cma: Add vc_cma driver to enable use of CMA

Signed-off-by: popcornmix <popcornmix@gmail.com>

vc_cma: Make the vc_cma area the default contiguous DMA area

vc_cma: Provide empty functions when module is not built

Providing empty functions saves the users from guarding the
function call with an #if clause.
Move __init markings from prototypes to functions.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>