sched: walt: Bring back cpufreq based current frequency tracking

When cycle counter is not available, we have to depend on the
cpufreq transition notifiers to track the current frequency.
This support is removed during HMP and WALT refactoring under
the assumption that it is not needed any more. We have some
upcoming targets which don't have cycle counter support, so
bring this feature back.

The sched_get_task_cpu_cycles() trace point is currently broken
as we are not tracking the current frequency based on cpufreq.
When cycle counter is available, store the current frequency but
skip the update_task_ravg() calls.

Change-Id: I3d900693d6cdb59a4756a44ccaefcd9e6df75c77
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>