iommu/arm-smmu: workaround some ATOS hw errata

Thulium has a hardware errata that requires that the SMMU be halted and
that a TLBIALL be issued before the ATOS command.  Add a DT option that
implements this workaround.

Change-Id: Ic40c7b93d64eebb97fe77082d8335debab624af1
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>