arm64: use swiotlb_alloc and swiotlb_free

The generic swiotlb_alloc and swiotlb_free routines already take care
of CMA allocations and adding GFP_DMA32 where needed, so use them
instead of the arm specific helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>