iommu/iommu-debug: Add file for profiling fast mapper

We'd like to understand the performance of the fast page table mapper,
which only supports 4K page sizes.  Add a debugfs file to profile the
new mapper.

CRs-Fixed: 997751
Change-Id: I5adc3c3ecd432552386b600b9e66e3db42e73138
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>