sched/cpufreq: Boost schedutil frequency ramp up

In some situations, it can be interesting to spend temporarily more
power if that can give a useful frequency boost.

The sugov_cpu_is_busy() heuristic is reused to check if there has been
some idle time on all CPUs in the considered perf domain since last call
to schedutil's get_next_freq(). If not, it is assumed that at least one
CPU is in a frequency ramp up phase and the domain will be allowed to
spend extra power to reach a stable OPP in a shorter amount of time.

Since the extra power expenditure is bounded, it cannot skyrocket even
on platforms with a large number of cores in the same frequency domain
and/or very high ratio between lowest and highest OPP cost.

Signed-off-by: Douglas RAILLARD <douglas.raillard@arm.com>