xen/arm: remove xen_dma_ops

arm and arm64 can just use xen_swiotlb_dma_ops directly like x86, no
need for a pointer indirection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>