powerpc/pci: Drop XIVE restriction on MSI domains

The PowerNV and pSeries platforms now have support for both the XICS
and XIVE IRQ domains.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701132750.1475580-23-clg@kaod.org