iommu: io-pgtable: add .map_sg op to io_pgtable_ops

The default implementation of map_sg (provided by default_iommu_map_sg
from the IOMMU API layer) actually just calls iommu_map repeatedly.
Often times there are locks or other initial setup in the implementation
of iommu_map, so there's opportunity to drastically improve performance
by factoring that out of the loop.  For this reason, io-pgtable
providers might want to implement .map_sg.  Add the declaration to the
io_pgtable_ops structure.

Change-Id: I5aa5e4e24a68e303ce2c005dc1dd0b33d5635ab3
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>