iommu: arm-smmu: Use subsys_initcall to register driver

Upstream supports early driver registration through IOMMU_OF_DECLARE.
However, since our clock and regulator dependencies are not available at
that point the driver would defer.

Use the normal method of driver registration through an initcall instead.

Change-Id: I3083c92324550fa001b222e0fb0efaafae8f197c
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>