OPP: Extend dev_pm_opp_data with a level

Let's extend the dev_pm_opp_data with a level variable, to allow users to
specify a corresponding level (performance state) for a dynamically added
OPP.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>