powerpc/mpc83xx: Power Management support

Basic PM support for 83xx.  Standby is implemented as sleep.
Suspend-to-RAM is implemented as "deep sleep" (with the processor
turned off) on 831x.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>