dt-bindings: arm-smmu: Add binding doc for Qcom smmu-500

Qcom's implementation of arm,mmu-500 works well with current
arm-smmu driver implementation. Adding a soc specific compatible
along with arm,mmu-500 makes the bindings future safe.
Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/devicetree/bindings/iommu/arm,smmu.txt

@@ -27,6 +27,10 @@ conditions.
                   "qcom,msm8996-smmu-v2", "qcom,smmu-v2",
                   "qcom,sdm845-smmu-v2", "qcom,smmu-v2".
 
+                  Qcom SoCs implementing "arm,mmu-500" must also include,
+                  as below, SoC-specific compatibles:
+                  "qcom,sdm845-smmu-500", "arm,mmu-500"
+
 - reg           : Base address and size of the SMMU.
 
 - #global-interrupts : The number of global interrupts exposed by the