pseries: Don't allow TCE (iommu) tables to be registered with duplicate LIOBNs

The PAPR specification requires that every bus or device mediated by the
IOMMU have a unique Logical IO Bus Number (LIOBN).  This patch adds a check
to enforce this, which will help catch errors in configuration earlier.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
