sparc32: use per-device dma_ops

sparc32 is the last platform making dynamic decisions in
get_arch_dma_ops based on the bus passed in.  Instead set the
iommu dma_ops at iommu probing and propagate them in
of_propagate_archdata, falling back to the NULL ops for the
direct mapping in the Leon or PCI case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>