sched: add support for CPU frequency estimation with cycle counter

At present scheduler calculates task's demand with the task's execution
time weighted over CPU frequency.  The CPU frequency is given by
governor's CPU frequency transition notification.  Such notification
may not be available.

Provide an API for CPU clock driver to register callback functions so
in order for scheduler to access CPU's cycle counter to estimate CPU's
frequency without notification.  At time point scheduler assumes the
cycle counter increases always even when cluster is idle which might
not be true.  This will be fixed by subsequent change for more accurate
I/O wait time accounting.

CRs-fixed: 1006303
Change-Id: I93b187efd7bc225db80da0184683694f5ab99738
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>