powerpc/pseries: Move MSI-related ops to pci_controller_ops

Move the pseries platform to use the pci_controller_ops structure
rather than ppc_md for MSI related PCI controller operations

We need to iterate all PHBs because the MSI setup happens later than
find_and_init_phbs() - mpe.

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