powerpc: Create pci_controller_ops.probe_mode and shim

Add pci_controller_ops.probe_mode, shadowing ppc_md.pci_probe_mode.
Add a shim, and changes the callsites to use the shim.

We also need to move the probe mode defines to pci-bridge.h from pci.h.
They are required by the shim in order to return a sensible default.
Previously, the were defined in pci.h, but pci.h includes pci-bridge.h
before the relevant #defines. This means the definitions are absent
if pci.h is included before pci-bridge.h. This occurs in some drivers.
So, move the definitons now, and move them back when we remove the shim.

Anything that wants the defines would have had to include pci.h, and
since pci.h includes pci-bridge.h, nothing will lose access to the
defines.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>