[POWERPC] Make function of pm_power_off consistent with x86

Allow the pm_power_off function variable in PPC to work as an override.
This makes the function consistent with the other architectures and it
allows generic poweroff operations (like those provided in IPMI
systems) to work properly on PPC.

Signed-off-by: Corey Minyard <minyard@acm.org>
Cc: Joseph Barnett <jbarnett@motorola.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>