iommu: Add DMA mapper for io-pgtable-fast

io-pgtable-fast does some underhanded tricks to achieve performance.
One of those tricks is that it expects clients to call its map function
directly, rather than going through the IOMMU framework.  Add a DMA API
implementation that goes through io-pgtable-fast.

CRs-Fixed: 997751
Change-Id: Iebcafeb630d9023f666078604898069e9f26dfdd
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>