ARM: dts: msm: Add iommu coherent test device for SDM670

Add a new iommu test device which is dma-coherent. This
test device will be used for validating buffer coherency.

Change-Id: I4bcb3f6976b9de7f42bcfb413306792858cfef89
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>