powerpc/pci: Fix comments about ppc_md.pcibios_fixup

The patch fixes the comments about ppc_md.pcibios_fixup(), which
should be called after allocating resources.

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