thermal: core: Make struct thermal_zone_device definition internal

Move the definitions of struct thermal_trip_desc and struct
thermal_zone_device to an internal header file in the thermal core,
as they don't need to be accessible to any code other than the thermal
core and so they don't need to be present in a global header.

No intentional functional impact.

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>