PM / OPP: Create a directory for opp bindings

More platform specific extended opp bindings will follow and it would be
easy to manage them with a directory for opp. Lets create that and move
the existing opp bindings into it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>