sched/fair: add boosted CPU usage

The CPU usage signal is used by the scheduler as an estimation of the
overall bandwidth currently allocated on a CPU. When SchedDVFS is in
use, this signal affects the selection of the operating points (OPP)
required to accommodate all the workload allocated in a CPU.
A convenient way to boost the performance of tasks running on a CPU,
which is also little intrusive, is to boost the CPU usage signal each
time it is used to select an OPP.

This patch introduces a new function:
  get_boosted_cpu_usage(cpu)
to return a boosted value for the usage of a specified CPU.
The margin added to the original usage is:
  1. computed based on the "boosting strategy" in use
  2. proportional to the system-wide boost value defined by provided
     user-space interface

The boosted signal is used by SchedDVFS (transparently) each time it
requires to get an estimation of the capacity required for a CPU.

Change-Id: I92db4404eef236b736be8a9345e6e0018fbf489a
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>