iommu: iommu-debug: Mark domain as debug domain

Mark the domain assoaciated with iommu debug devices as debug
domain. This is needed for the right selection of SIDs for ATOS.

Change-Id: Ia885f0ecf4f26b6b70c8fbd598dbe3260d26a329
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>