This adds a hook into the powerpc pci code for pci_disable_device() calls.  The
generic code already provides a weak pcibios_disable_device() symbol, so we
just need to provide our own in powerpc and it'll get picked up.

This is passed directly to the phb controller ops, provided one exists.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>