sched: Provide knob to prefer mostly_idle over idle cpus

sysctl_sched_prefer_idle lets the scheduler bias selection of
idle cpus over mostly idle cpus for tasks. This knob could be
useful to control balance between power and performance.

Change-Id: Ide6eef684ef94ac8b9927f53c220ccf94976fe67
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>