iommu: arm-smmu: Handle multiple arm-smmu instances

There may be multiple instances of an iommu present on the same bus.
Ensure the add_device() callback is called for each iommu instance,
rather than just the first one.

Change-Id: Ia55ef6e8ed3bd85214d111bb29f4e22ca696b185
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>