ppc/pnv: fix XSCOM core addressing on POWER9

The XSCOM base address of the core chiplet was wrongly calculated. Use
the OPAL macros to fix that and do a couple of renames.

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