iommu/arm-smmu: prefer stage-1 mappings where we have a choice

For an SMMU that supports both Stage-1 and Stage-2 mappings (but not
nested translation), then we should prefer stage-1 mappings as we
otherwise rely on the memory attributes of the incoming transactions
for IOMMU_CACHE mappings.

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