[SPARC64]: Add SG merging support back into IOMMU code.

Mimicks almost perfectly the powerpc IOMMU code, except that it
doesn't have the IOMMU_PAGE_SIZE != PAGE_SIZE handling, and it also
lacks the device dma mask support bits.

I'll add that later as time permits, but this gets us at least back to
where we were beforehand.

Signed-off-by: David S. Miller <davem@davemloft.net>