powerpc/pic: Add a PIC_AP_INIT() to set up AP PIC info

[jhibbits]

Summary

pc_cpuid may not match the PIC's idea of a given CPU. Since openpic
has a WHOAMI register, we can use that to get the PIC's idea of the CPU.
This needs to be done on each AP, so add a PIC_AP_INIT device method so
the PIC can perform any AP-specific initialization at AP bootstrap time.

This fixes SMP on e6500, which is still lacking SMT support.