iommu/arm-smmu: set CBARn.BPSHCFG to NSH for s1-s2-bypass contexts

Whilst trying to bring-up an SMMUv2 implementation with the table
walker plumbed into a coherent interconnect, I noticed that the memory
transactions targetting the CPU caches from the SMMU were marked as
outer-shareable instead of inner-shareable.

After a bunch of digging, it seems that we actually need to program
CBARn.BPSHCFG for s1-s2-bypass contexts to act as non-shareable in order
for the shareability configured in the corresponding TTBCR not to be
overridden with an outer-shareable attribute.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>