powerpc/powernv: Check PHB type in advance

The patch checks PHB type a bit early to save a bit cycles
for P7 because we don't support M64 for P7IOC no matter what
OPAL firmware we have.

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