iommu/arm-smmu: support buggy implementations with secure cfg accesses

In such a case we have to use secure aliases of some non-secure
registers.

This handling is switched on by DT property
"calxeda,smmu-secure-config-access" for an SMMU node.

Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
[will: merged with driver option handling patch]
Signed-off-by: Will Deacon <will.deacon@arm.com>