cpufreq: exynos5440: Protect OPP search calls with RCU lock

As per the OPP library documentation(Documentation/power/opp.txt) all
OPP find/get calls should be protected by RCU locks.

Signed-off-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>