St. Thomas Aquinas addresses this in his Summa Theologica I q. 19 a. 1 article, "Whether God has free-will?" He answers (c.):
We have free-will with respect to what we will not of necessity or of natural instinct. For our will to be happy does not appertain to free-will, but to natural instinct. Hence other animals, that are moved to act by natural instinct, are not said to be moved by free-will. Since then God necessarily wills His own goodness, but other things not necessarily, as shown above (a. 3), He has free will with respect to what He does not necessarily will.