iommu: arm-smmu: Ensure tbu driver is registered before arm-smmu

Since the arm-smmu driver may depend on the tbu driver, ensure the later
is available to prevent probe deferral.

Change-Id: I0fd38c945e8fc6b60a7529c776fbebf1e5ccb0d5
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>