gpu: ion: add CMA heap

New heap type ION_HEAP_TYPE_DMA where allocation is done with dma_alloc_coherent API.
device coherent_dma_mask must be set to DMA_BIT_MASK(32).
ion_platform_heap private field is used to retrieve the device linked to CMA,
if NULL the default CMA area is used.
ion_cma_get_sgtable is a copy of dma_common_get_sgtable function which should
be in kernel 3.5

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Change-Id: If4b1a3f9c8a6bd72053226208832f4971e44372f