cpufreq: ondemand: Calculate gradient of CPU load to early increase frequency

Instead of checking only the absolute value of CPU load_freq to increase
frequency, we detect forthcoming CPU load rise and increase frequency
earlier.

Every sampling rate, we calculate the gradient of load_freq. If it is
too steep we assume that the load most probably will go over
up_threshold in next iteration(s) and we increase frequency immediately.

New tuners are introduced:
- early_demand: to enable this functionality (disabled by default).
- grad_up_threshold: over this gradient of load we will increase
frequency immediately.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>