[power] Improve suspend sequence.

[power] Improve suspend sequence.

- Expand pm_power_down() into struct pm_ops, with ->prepare(), ->enter()
  and ->finish() methods, so the platform drivers get called to do start
  and stop work during suspend sequence.

- Make sure devices are suspended/resumed in enter_state(), and that they
  are powered down in pm_suspend_mem() and pm_suspend_standby().

- Call ->prepare() in suspend_prepare() and ->finish() in suspend_finish().