powerpc/powernv: Allow to reserve one PE for multiple times

The PE numbers are reserved according to root port's M64 window,
which is aligned to M64 segment finely. So one PE shouldn't be
reserved for multiple times. We will reserve PE numbers according
to the M64 BARs of PCI device in subsequent patches, which aren't
aligned to M64 segment size finely. It means one particular PE
could be reserved for multiple times.

The patch allows one PE to be reserved for multiple times and we
print the warning message at debugging level.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>