sched: update sched_task_load trace event

Add best_cpu and latency field to sched_task_load trace event.  The latency
field represents combined latency of update_task_ravg(), update_task_ravg()
and select_best_cpu() which is useful to analyze latency overhead of HMP
scheduler.

Change-Id: Ie6d777c918d0414d361d758490e3cd7d509f5837
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>