iommu/arm-smmu: Skip the execution of CMD_PREFETCH_CONFIG

Hisilicon SMMUv3 devices treat CMD_PREFETCH_CONFIG as a illegal command,
execute it will trigger GERROR interrupt. Although the gerror code manage
to turn the prefetch into a SYNC, and the system can continue to run
normally, but it's ugly to print error information.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
[will: extended binding documentation]
Signed-off-by: Will Deacon <will.deacon@arm.com>

Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt

@@ -35,3 +35,6 @@ the PCIe specification.
 
                       NOTE: this only applies to the SMMU itself, not
                       masters connected upstream of the SMMU.
+
+- hisilicon,broken-prefetch-cmd
+                    : Avoid sending CMD_PREFETCH_* commands to the SMMU.