iommu: iommu-debug: Fix duplicate iommu_attachment entries

iommu_attach() and iommu_detach() may be called multiple times for the
same group/domain pair. Ensure that only a single entry is put on the
attachment list.

Change-Id: I9c6c106647fab4b63a3332b7a7181f70b75ba18f
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>