ppc/pnv: add a PSI bridge class model

To ease the introduction of the PSI bridge model for POWER9, abstract
the POWER chip differences in a PnvPsi class model and introduce a
specific Pnv8Psi type for POWER8. POWER8 interface to the interrupt
controller is still XICS whereas POWER9 uses the new XIVE model.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190307223548.20516-2-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>