iommu/arm-smmu: Add missing iommu_device_unregister()

The remove() callback for the ARM SMMU driver should ensure
the SMMU devices that are being registered are no longer
visible to the IOMMU framework, so remove the devices
by invoking iommu_device_unregister().

Change-Id: I87dd7b4619e5b26b7eed64b04e3716f3b9c417c5
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>