[power] Add initial support for suspend-to-disk.

[power] Add initial support for suspend-to-disk.

- Add prototypes for split-up swsusp functions that can be called from PM
  core:

  save()
  write()
  read()
  restore()
  free()

- Base PM core algorithm on existence of these functions, and allow
  graceful failure at each step.

- Add pm_resume() function to be called on init to attempt to read swsusp
  image.

- Add debugging.

- Make sure we can handle all modes for suspend-to-disk.

- Add empty split-up functions to swsusp.