On POWER9 systems, PHB controllers signal the XIVE interrupt controller
of a source interrupt notification using a store on a MMIO region. Add
traces for such events.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20210126171059.307867-2-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>