Earlier work (40bcad56f - powerpc/pic: Add a PIC_AP_INIT() to
set up AP PIC info) broke booting my dual G5 powermac.

After much digging, jhibbits@ and I discovered that the openpic
implementation for the memory/bus controller used in the G5 CPUs
doesn't implement /all/ of the openpic specification.
Notably it sticks the WHOAMI register in a different location.
This is reading 0x0 back for all the PICs which is .. very not great.

So to restore the previous behaviour, use a quick for now that jhibbits@
can set appropriately to trust WHOAMI.

I've tested this on my dual G5 PPC and it boots/runs fine.

Fixes: 40bcad56f