iommu: iommu-debug: Use iommu_attach_group instead

Multiple device may be in the same iommu group. In this case
iommu_attach_group must be used instead of iommu_attach_device.
Additionally, check for any existing domains for the group prior
to attaching a new domain, since overwriting an existing domain
may cause an iommu fault.

Change-Id: I4e042ae0a1fe927e8f12f999c1393b5f0526cf4d
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>