sched/cpufreq: uclamp: Add utilization clamping for FAIR tasks

Each time a frequency update is required via schedutil, a frequency is
selected to (possibly) satisfy the utilization reported by each
scheduling class. However, when utilization clamping is in use, the
frequency selection should consider userspace utilization clamping
hints.  This will allow, for example, to:

 - boost tasks which are directly affecting the user experience
   by running them at least at a minimum "requested" frequency

 - cap low priority tasks not directly affecting the user experience
   by running them only up to a maximum "allowed" frequency

These constraints are meant to support a per-task based tuning of the
frequency selection thus supporting a fine grained definition of
performance boosting vs energy saving strategies in kernel space.

Add support to clamp the utilization and IOWait boost of RUNNABLE FAIR
tasks within the boundaries defined by their aggregated utilization
clamp constraints.
Based on the max(min_util, max_util) of each task, max-aggregated the
CPU clamp value in a way to give the boosted tasks the performance they
need when they happen to be co-scheduled with other capped tasks.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

---
Changes in v6:
 Message-ID: <20181107113849.GC14309@e110439-lin>
 - sanity check util_max >= util_min
 Others:
 - wholesale s/group/bucket/
 - wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs