Update cpu hotplug framework for hotplug rtas events.

This patch updates the cpu hotplgu framework so that we can do cpu hotplug
from the probe/release sysfs files (the current method for user space
driven hotplug) or use rtas hotplug events to do hotplug completely in
the kernel.

NOTE: This is completely untested. Use at your own risk.

or not.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>