PM / OPP: add some lockdep annotations

Certain OPP APIs need to be called under RCU lock; let's add a few
rcu_lockdep_assert() calls to warn about potential misuse.

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