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

Previously, MSI ops were added to ppc_md at the subsys level. However,
in fsl_pci.c, PCI controllers are created at the at arch level. So,
unlike in e.g. PowerNV/pSeries/Cell, we can't simply populate a
platform-level controller ops structure and have it copied into the
controllers when they are created.

Instead, walk every phb, and attempt to populate it with the MSI ops.
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>