powerpc/powernv: Remove misleading comment in pci.c

PCI in powernv now supports quite a bit more than p5ioc2, so remove the
outdated comment.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>