ppc/pnv: Add cut down PSI bridge model and hookup external interrupt

ppc/pnv: Add cut down PSI bridge model and hookup external interrupt

The Processor Service Interface (PSI) Controller is one of the engines
of the "Bridge" unit which connects the different interfaces to the
Power Processor.

This adds just enough of the PSI bridge to handle various on-chip and
the one external interrupt. The rest of PSI has to do with the link to
the IBM FSP service processor which we don't plan to emulate (not used
on OpenPower machines).

The ics_get() and ics_resend() handlers of the XICSFabric interface of
the PowerNV machine are now defined to handle the Interrupt Control
Source of PSI. The InterruptStatsProvider interface is also modified
to dump the new ICS.

Originally from Benjamin Herrenschmidt <benh@kernel.crashing.org>

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