powerpc: use device_on|offline() in rtas migration

During PowerVM LPAR migration, all offline cpus are brought back
online, and then taken back offline after migration is complete.
This is currently done with cpu_down() and cpu_up(), but should
be done with device_offline() and device_online().

Change powerpc rtas migration code to use device_offline() and
device_online() instead of directly calling cpu_down() and cpu_up().

Signed-off-by: Dan Streetman <ddstreet@ieee.org>