powerpc/ppc4xx_msi: Move MSI-related ops to pci_controller_ops

Move the ppc4xx msi subsystem to use the pci_controller_ops structure
rather than ppc_md for MSI related PCI controller operations.

As with fsl_msi, operations are plugged in at the subsys level, after
controller creation. Again, we iterate over all controllers and
populate them with the MSI ops.

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