iommu/iommu-debug: Provide debugfs for iommu_reg_{read,write}

It can be useful to read and modify SMMU settings interactively.  Add a
debugfs interface for the recently added iommu_reg_read and
iommu_reg_write functions for this purpose.

Change-Id: If3c9bf1833dc950c7ca1e2ce1a373bcaa31ebcf0
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>