powerpc/pseries/pci: Introduce __find_pe_total_msi()

It will help to size 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/20210701132750.1475580-2-clg@kaod.org