ARM: dts: msm: Add iommu coherent test device for SDM845

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

Change-Id: I673009d1445e9d0c04c3a42bab91f7eb87510b9f
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>