powerpc/pseries/pci: Introduce rtas_prepare_msi_irqs()

This splits the routine setting the MSIs in two parts: allocation of
MSIs for the PCI device at the FW level (RTAS) and the actual mapping
and activation of the IRQs.

rtas_prepare_msi_irqs() will serve as a handler for the PCI MSI domain.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-3-clg@kaod.org