pnv/psi: Allow access to PSI registers through xscom

pnv/psi: Allow access to PSI registers through xscom

skiboot only uses mmio to access the PSI registers (once the BAR is
set) but we don't have any reason to block the accesses through
xscom. This patch enables xscom access to the PSI registers. It
converts the xscom addresses to mmio addresses, which requires a bit
of care for the PSIHB, then reuse the existing mmio ops.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-ID: <20230630102609.193214-1-fbarrat@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>