OPP: Add generic key finding helpers and use them for freq APIs

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>