[POWERPC] powermac: Support G5 CPU hotplug

This allows "hotplugging" of CPUs on G5 machines.  CPUs that are
disabled are put into an idle loop with the decrementer frequency set
to minimum.  To wake them up again we kick them just like when bringing
them up.  To stop those CPUs from messing with any global state we stop
them from entering the timer interrupt.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>