iommu/arm-smmu: Set SMTNMB_TLBEN in ACR to enable caching of bypass entries

The SMTNMB_TLBEN in the Auxiliary Configuration Register (ACR) provides an
option to enable the updation of TLB in case of bypass transactions due to
no stream match in the stream match table. This reduces the latencies of
the subsequent transactions with the same stream-id which bypasses the SMMU.
This provides a significant performance benefit for certain networking
workloads.

With this change substantial performance improvement of ~9% is observed with
DPDK l3fwd application (http://dpdk.org/doc/guides/sample_app_ug/l3_forward.html

)
on NXP's LS2088a platform.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Nipun Gupta <nipun.gupta@nxp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>