sched: window-stats: synchronize windows across cpus

Synchronizing windows across cpus for task load measurements
simplifies cpu busy time accounting during migrations. For task
migrations, its usage in current window can be carried over to its new
cpu. This lets cpufreq governor see a correct picture of cpu busy time
that is not affected by migrations.

This patch lines up windows across cpus. One of the cpu, sync_cpu,
serves as a reference for all others. During bootup sync_cpu would
initialize its window_start (from its sched_clock()). Other cpus will
synchronize their window_start in reference to sync_cpu. This patch
assumes synchronous sched_clock() across cpus and may need some change
to address architectures which do not provide such synchronized
sched_clock().

Change-Id: I13381389a72f5f9f85cc2446401d493a55c78ab7
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>