arm64: use the generic swiotlb_dma_ops

Now that the generic swiotlb code supports non-coherent DMA we can switch
to it for arm64.  For that we need to refactor the existing
alloc/free/mmap/pgprot helpers to be used as the architecture hooks,
and implement the standard arch_sync_dma_for_{device,cpu} hooks for
cache maintaincance in the streaming dma hooks, which also implies
using the generic dma_coherent flag in struct device.

Note that we need to keep the old is_device_dma_coherent function around
for now, so that the shared arm/arm64 Xen code keeps working.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>