sched: Introduce the 'sched entity PELT' tracepoint

To let this tracepoint work on all configurations, the following special
handling is necessary for non-existent key=value
pairs:

id = -1      : In case sched entity is representing a task
             : In case of 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>