There are three type of helpers, to find exact, ceil, and floor values,
replicated for multiple key types, freq, level, bw. And all of these
helpers share a lot of boilerplate code.

Add generic key finding helpers to reduce code redundancy.

Also update the freq finder helpers to use the new infrastructure.
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>