most chargers have an airgap to allow airflow, and free flow of air around any transformer (which at it's heart, what a charger really is) can be very important, for cooling, but I would expect the mounting standards to be manufacturer's design specific, and not really something ANSI would pay a lot of attention towards.
What brings up the question? I have seen plenty of chargers bolted to the floor, in plants that I knew were often inspected for all sorts of safety concerns.