[POWERPC] Always hookup PHB IO resource even when empty

We must always hookup the pci_bus resource 0 to the PHB io_resource
even if the latter is empty (the bus has no IO support).  Otherwise,
some other code will end up hooking it up to something bogus and the
resource tree will end up being broken.

This fixes boot on QS20 Cell blades where the IDE driver failed to
allocate the IO resources due to breakage of the resource tree.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>