sched: walt: Use cpufreq transition notifier selectively

CPU frequency fast switching is disallowed if there are any cpufreq
transition notification listeners. cpufreq transition notifier is
needed in the scheduler in the absence of cycle counter support.
Unregister cpufreq transition notifier upon clock driver registering
cycle counter callback with the scheduler.

Change-Id: Ib19444be639a96589d0c3b29900b997c228df595
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>