ARM: dts: msm: Rename iommu test devices for sdm845

The device tree name is used to create a file in debugfs. Having a file
named "qcom,iommu-test-device" did not indicate which iommu instance
this file represented.

Additionally, update the SID used for the kgsl_iommu_test_device to 0x7.

Change-Id: If6c003aceceef28b340e3f46d13f653116e98b16
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>