OPP: Introduce dev_pm_opp_find_freq_ceil_by_volt()

This patch introduces a new helper routine in the OPP core, which
returns the OPP with the highest frequency which has voltage less than
or equal to the target voltage passed to the helper.

Signed-off-by: Andrew-sh.Cheng <andrew-sh.cheng@mediatek.com>
[ Viresh: Massaged the commit log and renamed the helper with some
cleanups. ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>