powerpc: use device_on|offline() instead of cpu_up|down()

In powerpc pseries platform dlpar operations, Use device_online() and
device_offline() instead of cpu_up() and cpu_down().

Calling cpu_up/down directly does not update the cpu device offline
field, which is used to online/offline a cpu from sysfs.  Calling
device_online/offline instead keeps the sysfs cpu online value correct.

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