thermal: core: Add function to walk trips under zone lock

Add a wrapper around for_each_thermal_trip(), called
thermal_zone_for_each_trip(), that will invoke the former under the
thermal zone lock and pass its return value to the caller.

Two drivers will be modified subsequently to use this new function.

No functional impact.

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