drivers: cpuidle: lpm-levels: Switch to new hotplug state callbacks

The hotplug framework has moved away from using the notifications for
hotplug events. With the new state based mechanism, the drivers are
required to register for explicit callbacks associated with driver
specifics hotplug states.

Switch to using new hotplug state callbacks.

Change-Id: Ic634a41b336decdbcfe5c4fd22ab2bd25c53b340
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>