arm64: tegra: Use correct compatible string for Tegra186 SMMU

The SMMU found on Tegra186 requires interoperation with the memory
controller in order to program stream ID overrides. The generic ARM SMMU
500 compatible is therefore inaccurate. Replace it with a more correct,
SoC-specific compatible string.

Signed-off-by: Thierry Reding <treding@nvidia.com>