thermal: core: Add governor callback for thermal zone change

Add a new callback to the struct thermal_governor. It can be used for
updating governors when there is a change in the thermal zone internals,
e.g. thermal cooling device is bind to the thermal zone.

That makes possible to move some heavy operations like memory allocations
related to the number of cooling instances out of the throttle() callback.

Both callback code paths (throttle() and update_tz()) are protected with
the same thermal zone lock, which guaranties the consistency.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>