[POWERPC] pseries: Enabling auto poweron after power is restored.

During power outages, the UPS notifies the system for a shutdown.
In the current setup, it isn't possible to poweron when power is
restored.  This patch fixes the issue by calling the right
ibm,power-off-ups token during such events.  It also adds a sysfs
interface so userspace can specify whether or not to power on when
power is restored.

Signed-off-by: Manish Ahuja <ahuja@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>