iommu: Create iommu debugfs directory from IOMMU code

Currently we're creating an "iommu" debugfs directory from the
iommu-debug code.  Other IOMMU modules might want to make use of this
same directory, so create it from the IOMMU framework code itself.

Change-Id: I679fdfc34ba5fcbd927dc5981438c6fabcfa3639
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>