sched/events: Introduce cfs_rq PELT trace event sched_pelt_cfs_rq()

The trace event is only defined if symmetric multi-processing
(CONFIG_SMP) is enabled.
To let this trace event work for configurations with and without group
scheduling support for SCHED_OTHER (CONFIG_FAIR_GROUP_SCHED) the
following special handling is necessary for non-existent key=value
pairs:

id = -1 : In case of !CONFIG_FAIR_GROUP_SCHED the task group css id is
          set to -1.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>