iommu/arm-smmu: Move the SMMU driver to subsys_initcall

Change the initcall level for the SMMU driver to subsys_initcall
so that the IOMMU devices get probed earlier in the bootup
sequence. This helps reduce the number of probe retrials for
client drivers that depend on an IOMMU.

Change-Id: If49b41c9b63f2eaf75cd6b7460b1d4ffc10e56db
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>