dt-bindings: iommu: qcom,iommu: Add MSM8953 GPU IOMMU to SMMUv2 compatibles

Add MSM8953 compatible string with "qcom,msm-iommu-v2" as fallback
for the MSM8953 GPU IOMMU which is compatible with Qualcomm's secure
fw "SMMU v2" implementation.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>