cpufreq: add generic get_cpuidle_time to the cpufreq headers so that we don't need to implement this everytime we need it.

Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>
cpufreq: conservative: fix compilation due to using the generic cpu_get_idle_time()

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
cpufreq: create a helper function to get CPU utilization. Picked from @stratosk

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>