arm64: Hook up IOMMU dma_ops

With iommu_dma_ops in place, hook them up to the configuration code, so
IOMMU-fronted devices will get them automatically.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>