powerpc: Add MSI operations to pci_controller_ops struct

Add MSI setup and teardown functions to pci_controller_ops.

Patch the callsites (arch_{setup,teardown}_msi_irqs) to prefer the
controller ops version if it's available.

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