drivers: Add option to reserve default CMA region

CMA provides good utilization of memory but for some use cases, the
allocation time is too costly. Add a Kconfig option to allow the
default region to be permanently set aside for contiguous use cases.

Change-Id: I1eef508d37cf6ae3b7b7a652fc59391b186fc122
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>