powerpc: Initialize the Hardware Interrupt Offset Register (HIOR) earlier for ppc970

Since we now have a much larger KVA on powerpc64, it's possible to get SLB
traps earlier in boot, possibly even before the HIOR is properly configured
for us. Move the HIOR setup to immediately after reset, so that we use our
exception handlers instead of Open Firmware's.

PR: 233863
Submitted by: Mark Millard (partial)
Reported by: Mark Millard
MFC after: 2 weeks