ARM: dts: msm: Add iommus property for iommu device for msmcobalt

The iommu test framework relies on at least one client being present
in device tree. Treat the iommu device itself as this client.

Change-Id: I373ad3648d0ee2136e787677e9d3860bb1599a83
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>