powerpc/powernv/pci: Add MSI domains

This is very similar to the MSI domains of the pSeries platform. The
MSI allocator is directly handled under the Linux PHB in the
in-the-middle "PNV-MSI" domain.

Only the XIVE (P9/P10) parent domain is supported for now. Support for
XICS will come 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/20210701132750.1475580-13-clg@kaod.org