sched/pelt: Introduce PELT multiplier

The new sysctl sched_pelt_multiplier allows a user to set a clock
multiplier to x2 or x4 (x1 being the default). This clock multiplier
artificially speeds up PELT ramp up/down similarly to use a faster
half-life than the default 32ms.

  - x1: 32ms half-life
  - x2: 16ms half-life
  - x4: 8ms  half-life

Internally, a new clock is created: rq->clock_task_mult. It sits in the
clock hierarchy between rq->clock_task and rq->clock_pelt.

[peterz: Use sched_feat()]
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>