sched: Add the sched_set_wake_to_idle API

Tasks may want to be placed on an idle CPU, overriding
regular task placement logic. Add the API that provides
this feature. It's upto the placement logic to actually
respect the new PF_* flag.

Change-Id: I0b39ecb8556e30ed30ff0c31ac683d92844573d5
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>