sched: walt: Provide the most recent window_start value to cpufreq

cpufreq may require rq->window_start for various reasons.
Also switch the timestamp passed to the governor from
sched_clock() to sched_ktime_clock() in order to ensure
that cpufreq can compare the timestamp to rq->window_start.

Change-Id: Icb78f8923a678f724b3e1101ef2d4248e7b2a796
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>