sched/events: Introduce sched entity PELT trace event sched_pelt_se()

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 sched entity is representing a task
             : In case CONFIG_FAIR_GROUP_SCHED is not set

pid = -1     : In case sched entity is representing a task group

comm = "n/a" : In case sched entity is representing a task group

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