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>