[power] Improve suspend functions.

[power] Improve suspend functions.

- Implement pm_suspend(), which is callable from anywhere in the kernel,
  and takes one of

  	PM_SUSPEND_STANDBY
	PM_SUSPEND_MEM
	PM_SUSPEND_DISK

  and enters the appropriate state.

- Change sysfs file to look for

  "standby"
  "mem"
  "disk"

  for what state to enter (rather than 'suspend' and 'hibernate' for the
  latter two).

- Add pm_sem to block multiple suspend sequences happening at once.

- Allocate a console and stop processes from common code before entering
  state.

- Add pm_power_down() callback for platform drivers to implement. Will be
  called to actually enter the low-power state.