arm64: dma-mapping: Split large pages when allocating in atomic context

In atomic context, gen_pool_alloc allocates a single page large
enough to accomodate the requested size. However __iommu_create_mapping
always maps pages assuming they are of size 4K. Thus only the first
4K of the buffer is mapped and a translation fault is generated
during an unmap.

Fix this by splitting the larger pages into 4K pages.

Change-Id: Ifcbe29477ad210204028486bd011470fe8b50852
Signed-off-by: Neeti Desai <neetid@codeaurora.org>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>