powerpc/powernv/pci: Introduce __pnv_pci_ioda_msi_setup()

It will be used as a 'compose_msg' handler of the MSI domain introduced
later.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-12-clg@kaod.org