OPP: Add key specific assert() method to key finding helpers

The helpers for the clock key, at least, would need to assert that the
helpers are called only for single clock case. Prepare for that by
adding an argument to the key finding helpers.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>