sched: Introduce CONFIG_SCHED_FREQ_INPUT

Introduce a compile time flag to enable scheduler guidance of
frequency selection. This flag is also used to turn on or off
window-based load stats feature.

Having a compile time flag will let some platforms avoid any
overhead that may be present with this scheduler feature.

Change-Id: Id8dec9839f90dcac82f58ef7e2bd0ccd0b6bd16c
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>