OPP: Add dev_pm_opp_find_freq_exact_indexed()

[ Upstream commit a5893928 ]

The indexed version of the API is added for other floor and ceil, add
the same for exact as well for completeness.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Stable-dep-of: b44b9bc7 ("OPP: fix dev_pm_opp_find_bw_*() when bandwidth table not initialized")
Signed-off-by: Sasha Levin <sashal@kernel.org>