soc: qcom: rpmpd: Add support for get/set performance state

Add support for the .set_performace_state() and .opp_to_performance_state()
callbacks in the rpmpd driver.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>