thermal/drivers/cpu_cooling: Add Software Package Data Exchange (SPDX)

For license auditing purpose, let's add the SPDX tag.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>