cpufreq: schedutil: move slow path from workqueue to SCHED_FIFO task

If slow path frequency changes are conducted in a SCHED_OTHER context
then they may be delayed for some amount of time, including
indefinitely, when real time or deadline activity is taking place.

Move the slow path to a real time kernel thread. In the future the
thread should be made SCHED_DEADLINE. The RT priority is arbitrarily set
to 50 for now.

Hackbench results on ARM Exynos, dual core A15 platform for 10
iterations:

$ hackbench -s 100 -l 100 -g 10 -f 20

Before			After
---------------------------------
1.808			1.603
1.847			1.251
2.229			1.590
1.952			1.600
1.947			1.257
1.925			1.627
2.694			1.620
1.258			1.621
1.919			1.632
1.250			1.240

Average:

1.8829			1.5041

Based on initial work by Steve Muckle.

Signed-off-by: Steve Muckle <smuckle.linux@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>