iommu/arm-smmu: Allow the ARM-SMMU driver to be a module

Allow the ARM SMMU driver to be compiled as a loadable
kernel module.

Change-Id: Ib982ddb7f8ed82c5f7983db8b85bb2c6c83502c6
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>