sched/cpufreq: Add the SPDX tags

The SPDX tags are not present in cpufreq.c and cpufreq_schedutil.c.

Add them and remove the license descriptions

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>