The USB2Dynamixel is the “official” way of connecting the AX-12A servos (and other servos in the Robotis family) to a computer like the Raspberry Pi. See http://www.tribotix.com/Products/Robotis/OtherStuff/Comms.htm for instance. They're not cheap, last retailing for $65.70 each. Drivers already exist for them though so if you need something that's just plug-and-play that might be the way to go.