U-Boot: PPC: Spice up CPU detection, trying both PVR and FDT

* Read the PVR register to determine the CPU type.
* Also check the FDT /cpu/cpu@0 node for model string
  (we use the FDT calls directly since the OF wrapper isn't initialized yet).
* Use this to enable the FPU APU hack for 440 (instead of #ifdef).