Certification of Aircraft Phone/Tablet Mounts/Holders
For general aviation no certification is required, neither is certification required for at least some commercial operations like flight training, at least for the regulatory bodies I'm familiar with (FAA and EASA). I've seen Ram and Garmin mounts being used on some smaller passenger and cargo operations as well, usually GPS and tablets in older prop airplanes like the DC3, Beechcraft twins and the like. 

As for carrier airline operations I am not sure.