sched: Add C-state tracking to the sched_cpu_load trace event

C-state information is used by the scheduler for small task placement
decisions. Track this information in the sched_cpu_load trace event.
Also add the trace event in best_small_task_cpu(). This will help
better understand small task placement decisions.

Change-Id: Ife5f05bba59f85c968fab999bd13b9fb6b1c184e
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>