sched: start tracking placement algorithm runtime

The efficiency of the placement algorithm is critical to the
performance of the scheduler. Start tracking this metric so that
the impact of optimizations and feature additions can easily be
assessed and verified.

Change-Id: Ie38670e05e341c6e0f898bb41ab4de13347f72ff
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>