msm: lpm: Support cluster low power modes for cpu hotplug

When all cpus in any cluster are powered down, we would want to do cluster
low power modes and save power draw by devices like L2, associated with the
cluster. Also, bubble up and allow low power modes on top level clusters.

Change-Id: I68bc2ccbb3cff884da9356ef9aad88d5c2207c10
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>