ppc/pnv: add dummy XSCOM registers for PRD initialization

PRD (Processor recovery diagnostics) is a service available on
OpenPower systems. The opal-prd daemon initializes the PowerPC
Processor through the XSCOM bus and then waits for hardware diagnostic
events.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190527071722.31424-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>