powerpc/powernv: Initialize M64 PE in time

The patch initializes PE instance when reserving PE number to
keep consistent things as we did before. Also, it replaces the
iteration on bridge's windows with the prefered way.

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