arm64: Add IOMMU dma_ops

Taking some inspiration from the arch/arm code, implement the
arch-specific side of the DMA mapping ops using the new IOMMU-DMA layer.

Since there is still work to do elsewhere to make DMA configuration happen
in a more appropriate order and properly support platform devices in the
IOMMU core, the device setup code unfortunately starts out carrying some
workarounds to ensure it works correctly in the current state of things.

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