OPP: Add dev_pm_opp_find_level_floor()

Add dev_pm_opp_find_level_floor(), as is done for frequency and
bandwidth.

Signed-off-by: Krishna chaitanya chundru <quic_krichai@quicinc.com>
[ Viresh: Updated commit log and rearranged code ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>