[POWERPC] MPC5200 low power mode

Low-power mode implementation for Lite5200b.
Some I/O registers are also saved here.

A recent U-Boot that supports this (lite5200b_PM_config) is needed.

Signed-off-by: Domen Puncer <domen.puncer@telargo.com>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>