[SPARC64]: Eliminate PCI IOMMU dma mapping size limit.

The hairy fast allocator in the sparc64 PCI IOMMU code
has a hard limit of 256 pages.  Certain devices can
exceed this when performing very large I/Os.

So replace with a more simple allocator, based largely
upon the arch/ppc64/kernel/iommu.c code.

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