thermal: core: Unexport thermal_bind_cdev_to_trip() and thermal_unbind_cdev_from_trip()

Since thermal_bind_cdev_to_trip() and thermal_unbind_cdev_from_trip()
are only called locally in the thermal core now, they can be static,
so change their definitions accordingly and drop their headers from
the global thermal header file.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Huisong Li <lihuisong@huawei.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://patch.msgid.link/3512161.QJadu78ljV@rjwysocki.net