iommu: io-pgtable-arm: implement .map_sg

Mapping an entire scatterlist at once is faster than calling iommu_map
on each link individually.  Implement .map_sg in the ARM LPAE page table
allocator so that drivers using the allocator can leverage this
performance boost.

Change-Id: I77f62a2566058693c3f58fc0b05d715a780ae5d8
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>