dt-bindings: arm-smmu: Document SM8650 GPU SMMU

Document the GPU SMMU found on the SM8650 platform.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/578685/


Signed-off-by: Rob Clark <robdclark@chromium.org>