thermal: core: Introduce thermal_trip_crossed()

Add a helper function called thermal_trip_crossed() to be invoked by
__thermal_zone_device_update() in order to notify user space, the
thermal debug code and the zone governor about trip crossing.

Subsequently, this will also be used in the case when a trip point
becomes invalid after being crossed on the way up.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>