perf sched: Make it easier to plug in new sub profilers

perf sched: Make it easier to plug in new sub profilers

Create a sched event structure of handlers in which various
sched events reader can plug their own callbacks.

This makes easier the addition of new perf sched sub commands.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>