PM / OPP: add dev_pm_opp_is_turbo() helper

Add dev_pm_opp_is_turbo() helper to verify if an opp is to be used only
for turbo mode or not.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>