sched/fair: add boosted task utilization

sched/fair: add boosted task utilization

The task utilization signal, which is derived from PELT signals and
properly scaled to be architecture and frequency invariant, is used by
EAS as an estimation of the task requirements in terms of CPU bandwidth.

When the energy aware scheduler is in use, this signal affects the CPU
selection. Thus, a convenient way to bias that decision, which is also
little intrusive, is to boost the task utilization signal each time it
is required to support them.

This patch introduces the new function:
  boosted_task_util(task)
which returns a boosted value for the utilization of the specified task.
The margin added to the original utilization is:
  1. computed based on the "boosting strategy" in use
  2. proportional to boost value defined either by the sysctl interface,
     when global boosting is in use, or the "taskgroup" value, when
     per-task boosting is enabled.

The boosted signal is used by EAS
  a. transparently, via its integration into the task_fits() function
  b. explicitly, in the energy-aware wakeup path

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>