iommu/arm-smmu: Enable bypass transaction caching for ARM SMMU 500

The ARM SMMU identity mapping performance was poor compared with the
DMA mode. It was found that enable caching would restore the performance
back to normal. The S2CRB_TLBEN bit in the ACR register would allow for
caching of the stream to context register bypass transaction information.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>