sched: take into account of limited CPU min and max frequencies

Actual CPU's min and max frequencies can be limited by hardware
components while governor's not aware of.  Provide an API for them to
notify for scheduler to be able to notice accurate currently
operating frequency boundaries which helps better task placement
decision.

CRs-fixed: 1006303
Change-Id: I608f5fa8b0baff8d9e998731dcddec59c9073d20
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>