ppc4xx: Add more PLB registers

These registers are present in 440 SoCs (and maybe in others too) and
U-Boot accesses them when printing register info. We don't emulate
these but add them to avoid crashing when they are read or written.

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