iommu/arm-smmu: allow translation stage to be forced on the cmdline

When debugging and testing code on an SMMU that supports nested
translation, it can be useful to restrict the driver to a particular
stage of translation.

This patch adds a module parameter to the ARM SMMU driver to allow this
by restricting the ability of the probe() code to detect support for
only the specified stage.

Signed-off-by: Will Deacon <will.deacon@arm.com>