PM / OPP: Assert RCU lock in exported functions

Add lockdep asserts for holding the RCU lock when calling
dev_pm_opp_get_freq() and dev_pm_opp_get_voltage() to aid in detecting
RCU misuses.

These are called often after dev_pm_opp_find_freq_ceil/exact() which
already asserts for RCU lock. However one could make an error by
releasing lock too early - just after dev_pm_opp_find_freq_ceil().

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 04bf1c7f76b05be8134a833ec023f1c96f81b8a1)
Signed-off-by: Alex Shi <alex.shi@linaro.org>