[SPARC64]: Fix powering off on SMP.

Doing a "SUNW,stop-self" firmware call on the other cpus is not the
correct thing to do when dropping into the firmware for a halt,
reboot, or power-off.

For now, just do nothing to quiet the other cpus, as the system should
be quiescent enough.  Later we may decide to implement smp_send_stop()
like the other SMP platforms do.

Based upon a report from Christopher Zimmermann.

Signed-off-by: David S. Miller <davem@davemloft.net>