On PowerPC 4xx or other non cache-coherent platforms, we lost the
appropriate cache flushing in dma_map_sg() when merging the 32 and
64-bit DMA code (commit 4fc665b88a79a45bae8bbf3a05563c27c7337c3d,
"powerpc: Merge 32 and 64-bit dma code"). This restores it.