PM / OPP: Manage device clk

OPP core has got almost everything now to manage device's OPP
transitions, the only thing left is device's clk. Get that as well.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit d54974c2513f487e9e70fbdc79c5da51c53e23da)
Signed-off-by: Alex Shi <alex.shi@linaro.org>