sched: disable frequency notifications by default

The frequency notifications from the scheduler do not currently respect
synchronous topologies. If demand on CPU 0 is driving frequency high and
CPU 1 is in the same frequency domain, and demand on CPU 1 is low,
frequency notifiers will be continuously sent by CPU 1 in an attempt to
have its frequency lowered.

Until the notifiers are fixed, disable them by default. They can still
be re-enabled at runtime.

Change-Id: Ic8a927af2236d8fe83b4f4a633b20a8ddcfba359
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>