cma: use MEMBLOCK_ALLOC_ANYWHERE for placing CMA regions

MEMBLOCK_ALLOC_ANYWHERE allocates blocks from anywhere,
including highmem. Use this flag to allow CMA regions to
be placed in highmem as opposed to lowmem.

Change-Id: Id5fa36a96e46d60f0e898d764a1f4c8a0a37f5f8
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>