sched: add scheduling latency tracking procfs node

Add a new procfs node /proc/sys/kernel/sched_max_latency_us to track the
worst scheduling latency.  It provides easier way to identify maximum
scheduling latency seen across the CPUs.

Change-Id: I6e435bbf825c0a4dff2eded4a1256fb93f108d0e
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>