ppc/pegasos2: Access MV64361 registers via their memory region

ppc/pegasos2: Access MV64361 registers via their memory region

Instead of relying on the mapped address of the MV64361 registers
access them via their memory region. This is not a problem at reset
time when these registers are mapped at the default address but the
guest could change this later and then the RTAS calls accessing PCI
config registers could fail. None of the guests actually do this so
this only avoids a theoretical problem not seen in practice.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <b6f768023603dc2c4d130720bcecdbea459b7668.1634241019.git.balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>