powerpc/powernv: Reserve the correct PE number

We're assigning PE numbers after the completion of PCI probe. During
the PCI probe, we had PE#0 as the super container to encompass all
PCI devices. However, that's inappropriate since PELTM has ascending
order of priority on search on P7IOC. So we need PE#127 takes the
role that PE#0 has previously. For PHB3, we still have PE#0 as the
reserved PE.

The patch supposes that the underly firmware has built the RID to
PE# mapping after resetting IODA tables: all PELTM entries except
last one has invalid mapping on P7IOC, but all RTEs have binding
to PE#0. The reserved PE# is being exported by firmware by device
tree.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>