thermal: gov_user_space: Use .trip_crossed() instead of .throttle()

Notifying user space about trip points that have not been crossed is
not particularly useful, so modify the User Space governor to use the
.trip_crossed() callback, which is only invoked for trips that have been
crossed, instead of .throttle() that is invoked for all trips in a
thermal zone every time the zone is updated.

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>