iommu: dma-mapping-fast: Add support for get_sgtable

The dma_get_sgtable() API is required to get the pages allocated via
dma_alloc_coherent() in the case where the device has an iommu, and
the allocated memory is not physically contiguous.

Change-Id: I47611032babe93662a4593605dbd646295910806
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>