sched: create a statistic structure

Create a statistic structure that will be used to share information with
other frameworks like cpuidle and cpufreq. This structure only contains the
current wake up latency of a core for now but could be extended with other
usefull information.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: flar2 <asegaert@gmail.com>