What are my options for plugging holes in soffit for security cameras?
You can plug the hole with exterior silicone caulking. It comes in squeeze tubes or caulking gun tubes. That's probably all you need.
For a more aesthetic finish, and a hard cover to prevent stubborn rodents to bite through (though unlikely needed), you can apply a an exterior cable cover. This is best done in one step, with the caulking still fresh and uncured.