arm/dt: iommu: Move property to derived nodes

Move boolean property from the base node to the derived
node so that targets that do not have an IOMMU with
this property can derive from the base IOMMU node.

Change-Id: I6eadada1c145d8b28d379d876b752e51c51db061
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>