cpufreq: cpu-boost: Use interruptible wait to not affect load average

Using the function wait_event in cpu_boost puts the
process enter to 'D' state which contribute to the
high load average. This change will put the process
boost_sync in the 'S' state (interruptible sleep).

CRs-Fixed: 655484
Change-Id: Ie121adbe1fac1d2862ac5342bb97c7c926f7d7a8
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>