sched: window-stats: ftrace event improvements

Add two new ftrace event:

* trace_sched_freq_alert, to log notifications sent
  to governor for requesting change in frequency.
* trace_sched_get_busy, to log cpu busytime information returned by
  scheduler

Extend existing ftrace events as follows:

* sched_update_task_ravg() event to log irqtime parameter
* sched_migration_update_sum() to log threadid which is being migrated
  (and thus responsible for update of curr_runnable_sum and
  prev_runnable_sum counters)

Change-Id: Ia68ce0953a2d21d319a1db7f916c51ff6a91557c
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>