How can I restrict only iPad to be connectable to USB port of Mac?
There are two basic solutions for restricting access to computer ports -- software-based, and hardware-based.

A software-based solution would probably be best in your case, but it might not be cheap.  There are security software packages available to block access to devices that are not on a "whitelist" that you establish.  One commercial example is Endpoint Protector.

Hardware-based solutions involve physically blocking access to certain ports.  The "meathead" way to do this is using epoxy.  People have done this, but it is generally not recommended as it is completely irreversible.  You would glue an iPad adapter cable into one port, then fill the other ports with a putty (to prevent epoxy from getting inside the computer where you don't want it) and glue over them.

There are a few USB port "locks" on the market that physically block USB ports -- for example, Kensington USB Port Lock ($15).  But you'd still have to epoxy an iPad adapter cable to your computer to allow iOS devices to connect while preventing access to other devices.