defconfig: msm: Enable the "fast" IOMMU page table mapper

It's fast for use cases that require super fast IOMMU mappings (in
exchange for memory).  Enable it.

CRs-Fixed: 997751
Change-Id: I016937309ac8e16775d13e63b630bb98469c9fca
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>