opp: Add dev_pm_opp_find_level_exact()

Since the performance states in the OPP table are unique, implement a
dev_pm_opp_find_level_exact() in order to be able to fetch a specific OPP.

Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
[ Viresh: Updated commit log ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>