iommu/iommu-debug: Show secure VMID in attach info

It can be useful to know a domain's secure VMID value.  Add this
information to the attach info debugfs file.

Change-Id: Ic8a73000b8337639f95376b13ef58a0e2a089dde
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>