drivers: thermal: cpu_cooling: Add a notifier for max level transitions

Add a blocking notifier to send a notification when a CPU enters or
leaves the max cooling level state. Also add an API to retrieve
the cpumask that contains all CPUs that are in max cooling state.

Interested clients like core_ctl subscribe to these notifications
and manage CPUs accordingly.

Change-Id: I4828ed572fa965b5bfce8184ef813fddfb246cb1
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[satyap@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>