gpu: ion: Add support for getting the heap size of CMA regions

Currently, Ion heaps are able to allocate memory from appropriate
CMA regions without knowing the maximum size of the CMA region. New
Ion algorithms necessitate having the size of the heap available
for accounting. Add code to parse the devicetree to get the size of
the CMA region and associate it with the appropriate heap.

Change-Id: Ia9b560d496c76209c6116fa00f24f7b624c954f3
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>