powerpc: Create pci_controller_ops.enable_device_hook and shim

Add pci_controller_ops.enable_device_hook,
shadowing ppc_md.pcibios_enable_device_hook.
Add a shim, and changes the callsites to use the shim.

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