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>
(cherry picked from commit 3566c5b277a41e912d15cd583c4604c95bb6b3f8)
Signed-off-by: Alex Shi <alex.shi@linaro.org>