sched: Provide an easy method to log context switch latencies

Allow logging of various sections of context switch in order to derive
the worst case latencies associated with them. This is required for
scheduler profiling.

Change-Id: I3a5009cb3088cc7ace2cd3130d4d7b24e957bada
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>