iommu/io-pgtable-arm: Check for v7s-incapable systems

On machines with no 32-bit addressable RAM whatsoever, we shouldn't
even touch the v7s format as it's never going to work.

Fixes: e5fc9753

 ("iommu/io-pgtable: Add ARMv7 short descriptor support")
Reported-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>