iommu: arm-smmu: Standardize atos disablement feature

Make this a devicetree property instead of tying it to a specific
iommu model. Future revisions of this iommu model may not have the
same issue.

Change-Id: I2274644db05b1967c8f368d99b74cd00fdeb1af2
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>