The threshold field in struct thermal_trip is only used internally by
the thermal core and it is better to prevent drivers from misusing it.
It also takes some space unnecessarily in the trip tables passed by
drivers to the core during thermal zone registration.

For this reason, introduce struct thermal_trip_desc as a wrapper around
struct thermal_trip, move the threshold field directly into it and make
the thermal core store struct thermal_trip_desc objects in the internal
thermal zone trip tables.  Adjust all of the code using trip tables in
the thermal core accordingly.

No intentional functional impact.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>