Restructure how reset and poweroff are handled on PowerPC systems, since

Restructure how reset and poweroff are handled on PowerPC systems, since
the existing code was very platform specific, and broken for SMP systems
trying to reboot from KDB.

- Add a new PLATFORM_RESET() method to the platform KOBJ interface, and
  migrate existing reset functions into platform modules.
- Modify the OF_reboot() routine to submit the request by hand to avoid
  the IPIs involved in the regular openfirmware() routine. This fixes
  reboot from KDB on SMP machines.
- Move non-KDB reset and poweroff functions on the Powermac platform
  into the relevant power control drivers (cuda, pmu, smu), instead of
  using them through the Open Firmware backdoor.
- Rename platform_chrp to platform_powermac since it has become
  increasingly Powermac specific. When we gain support for IBM systems,
  we will grow a new platform_chrp.