thermal: gov_step_wise: Use .manage() callback instead of .throttle()

Make the Step-Wise governor use the new .manage() callback instead of
.throttle().

Even though using .throttle() is not particularly problematic for the
Step-Wise governor, using .manage() instead still allows it to reduce
overhead by updating all of the cooling devices once after setting
target values for all of the thermal instances.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>