[PATCH] s390: add dummy pm_power_off

Define a dummy pm_power_off pointer to make sys_reboot happy.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>