[POWERPC] powerpc: Enable DEEPNAP power savings mode on 970MP

Without this patch, on an idle system I get:

cpu-power-0:21.638
cpu-power-1:27.102
cpu-power-2:29.343
cpu-power-3:25.784
Total: 103.8W

With this patch:

cpu-power-0:11.730
cpu-power-1:17.185
cpu-power-2:18.547
cpu-power-3:17.528
Total: 65.0W

If I lower HZ to 100, I can get it as low as:

cpu-power-0:10.938
cpu-power-1:16.021
cpu-power-2:17.245
cpu-power-3:16.145
Total: 60.2W

Another (older) Quad G5 went from 54W to 39W at HZ=250.

Coming back out of Deep Nap takes 40-70 cycles longer than coming back
from just Nap (which already takes quite a while). I don't think it'll
be a performance issue (interrupt latency on an idle system), but in
case someone does measurements feel free to report them.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>