OPP: Introduce dev_pm_opp_get_freq_indexed() API

In the case of devices with multiple clocks, drivers need to specify the
frequency index for the OPP framework to get the specific frequency within
the required OPP. So let's introduce the dev_pm_opp_get_freq_indexed() API
accepting the frequency index as an argument.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[ Viresh: Fixed potential access to NULL opp pointer ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>