iommu/iommu-debug: Don't add debugfs entries until we init

The attachment tracking code adds a node to debugfs every time a client
attaches a domain to an IOMMU device.  The problem is that clients can
start making those attachments during early boot, before iommu-debug
initializes (including setting up the root debugfs directory for the
attachment tracking), which means they get stuck in the root debugfs
directory.  Trying to initialize iommu-debug earlier than all possible
IOMMU clients is tricky, so fix this by only installing debugfs entries
onces we've initialized, installing debugfs entries for any early
attachments at initialization time.

Change-Id: I8364015346105187e0c8f787fc2b4155d72b3584
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>