[power] Register PM subsystem, and create power/ directory in sysfs.

[power] Register PM subsystem, and create power/ directory in sysfs.

- Add file 'state' which provides single point of entry for all PM
  transitions.
  File accepts a string specifying what state to enter, which is one of:
  "standby": Power-on Suspend (aka S1)
  "suspend": Suspend-to-RAM (aka S3)
  "hibernate": Suspend-to-disk (aka S4)

  The names do suck, because they are lifted from the ACPI spec. Better
  naming suggestions are welcome, though these names are relatively well-
  known.

- Handler for file does little now, but will validate the passed string and
  call the appropriate functions.

- Needs to be integrated with swsusp and ACPI S3 code.