cma: Add support for cma_get_size

HLOS assumes the size of the CMA based heaps is equal to the
size specified with the "linux-contiguous-region" property.
This is not always true, and CMA might reserve more memory
to take care of alignment. This causes the secure world to
lockdown more memory than it is supposed to, thus denying
access to clients. The cma_get_size api returns the correct
size which is reserved for these heaps, thus preventing
client access issues.

CRs-Fixed: 737584
Change-Id: Idf5f3587c0a2b1a300b2102079b6796323254cc8
Signed-off-by: Neeti Desai <neetid@codeaurora.org>