ppc/pnv: Add support for NMI interface

This implements the NMI interface for the PNV machine, similarly to
commit 34316482 ("spapr: Add support for new NMI interface") for
SPAPR.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20200325144147.221875-3-npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>