iommu/arm-smmu: select ARM64_DMA_USE_IOMMU

ARM64's dma-mapping.c is getting support for IOMMUs. Select
ARM64_DMA_USE_IOMMU for ARM64 so that the generic DMA APIs can leverage
IOMMU mappings.

Change-Id: I5adbac5073b6669f6f53680b8d57b7f8eeccc016
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>