iommu/arm-smmu: Unregister TBU driver when unloading the SMMU module

When the ARM SMMU module is unloaded, the exit routine does not
fully clean-up after the initialization routine, as it leaves
the TBU driver registered. Thus, unregister the TBU driver
when unloading the SMMU module.

Change-Id: I536f756dab7eebeffe0dd907629feed6657a2532
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>