iommu/iommu-debug: Add debugfs file to trigger context faults

It can be useful during development to trigger faults.  Add a debugfs
file to do so.

Change-Id: Ic7b304ef0d908ebd506979f0c87189e34d7dfc67
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>