iommu/arm-smmu: wire up .map_sg from the ARM LPAE io-pgtable

The ARM LPAE io-pgtable provider now has support for .map_sg.  Wire it
up.

Change-Id: I2eaea6f245e1059582f3bf04829e9e3f24675782
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>