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 was removed during HMP and WALT refactoring under
the assumption that it is not needed any more. However some
upcoming targets do not have cycle counter support, so bring
this feature back.

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