xen/iommu: smmu: Advertise when the SMMU support coherent table walk

xen/iommu: smmu: Advertise when the SMMU support coherent table walk

When SMMU doesn't support coherent table walk, Xen may need to clean
updated PT (see commit 4c5f4cb "xen/arm: p2m: Clean cache PT when the
IOMMU doesn't support coherent walk").

If one SMMU of the platform doesn't support coherent table walk, the
feature is disabled for the whole platform. This is because device is
assigned to a domain after the page table are populated.

This could impact performance on domain which doesn't use device
passthrough. But, as the spec strongly recommends the support of this
feature for mainstream platform, I expect server will always have SMMUs
supporting coherent table walk. If not, we may need to enable this feature
per-domain.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>