PM / OPP: Prefix exported opp routines with dev_pm_opp_

That's the naming convention followed in most of opp core, but few
routines didn't follow this, fix them.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>