iommu/iommu-debug: Move attachment info file to subdirectory

Currently IOMMU attachment info is available in debugfs files located at
<debugfs_root>/iommu/attachments/<attached_device>.  However, there are
more actions that can be taken on attached devices besides just printing
their info.  Make room for more debugfs files for attached devices by
creating a directory for each one, and move the existing info file to:
<debugfs_root>/iommu/attachments/<attached_device>/info.

Change-Id: Ia56efc3aeb5e82afc34314fe48aaa0cd6e5579be
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>