sched/cpufreq: Refactor the utilization aggregation method

Schedutil aggregates the PELT signals of CFS, RT, DL and IRQ in order
to decide which frequency to request. Energy Aware Scheduling (EAS)
needs to be able to predict those requests to assess the energy impact
of scheduling decisions. However, the PELT signals aggregation is only
done in schedutil for now, hence making it hard to synchronize it with
EAS.

To address this issue, introduce schedutil_freq_util() to perform the
aforementioned aggregation and make it available to other parts of the
scheduler. Since frequency selection and energy estimation still need
to deal with RT and DL signals slightly differently, schedutil_freq_util()
is called with a different 'type' parameter in those two contexts, and
returns an aggregated utilization signal accordingly.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>