Revert "iommu/arm-smmu: Make arm-smmu explicitly non-modular"

This reverts commit addb672f.

Let's get the SMMU driver building as a module, which means putting
back some dead code that we used to carry.

Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: John Garry <john.garry@huawei.com> # smmu v3
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>