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

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

Conflicts:

	drivers/gpu/ion/Makefile
	drivers/gpu/ion/ion_cma_heap.c
	drivers/gpu/ion/ion_heap.c
	drivers/gpu/ion/ion_priv.h
	include/linux/ion.h
Revert "gpu: ion: add CMA heap"

This reverts commit 80ae8ac6ff0164cb8b72cdba7fce62dd6945b132.