iommu/arm-smmu: Remove DOMAIN_ATTR_COHERENT_HTW_DISABLE

The DOMAIN_ATTR_COHERENT_HTW_DISABLE IOMMU domain attribute is
currently not being respected in any way.  SMMU coherency will be
configured through the SMMU device tree nodes moving forward.
Remove support for the obsolete option.

Change-Id: I3d3a939da588334b25951512b499fa14a9f08469
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>