kselftest: cpufreq: Add RTC wakeup alarm

Add RTC wakeup alarm for devices to resume after specific time interval.
This improvement in the test will help in enabling this test
in the CI systems and will eliminate the need of manual intervention
for resuming back the devices after suspend/hibernation.

Signed-off-by: Shreeya Patel <shreeya.patel@collabora.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>