iommu: dma-mapping-fast: Add mmap support

Clients may use dma_mmap_coherent to map buffers allocated through
dma_alloc_coherent() to userspace. Support this.

Change-Id: Ibf1f086be4c6692479c11cb4585954c5d3c91707
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>