Revert "iommu/arm-smmu: Set PRIVCFG in stage 1 STEs"

This reverts commit df5e1a0f2a2d779ad467a691203bcbc74d75690e.

Now that proper privileged mappings can be requested via IOMMU_PRIV,
unconditionally overriding the incoming PRIVCFG becomes the wrong thing
to do, so stop it.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>