thermal: trip: Define for_each_trip() macro

Define a new macro for_each_trip() to be used by the thermal core code
and thermal governors for walking trips in a given thermal zone.

Modify for_each_thermal_trip() to use this macro instead of an open-
coded loop over trips.

No intentional functional impact.

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