I said it because it was funny seeing that it's the same I call my grandmother, written yaya in Spanish. I looked it up before checking the comments and found some forums where they said that it came from Greek (via the Mediterranean, and then to America, where it's used in many countries), although the official dictionary says its origin is uncertain. I suppose we'll never know, but even if many languages have come to similar words independtly, I see some relation between the two, been so close (remember that the Catalan Company conquered the duchies of Neopatras and Athens and imposed the catalan as official language, even if not for a long time...).
In catalan we have yaya for grandmother and yayo (iaiou) for grandfather. It's an everyday word. The most formal word is avi (grandfather) and avia (grandmother).
The almogavars were mercenaries in the Middle Ages who came from Catalunya and other parts of Aragon. In a certain point they were hired by Greece and they left some words in their vocabulary. One of them is yaya. @Perriguez is right.
Btw,there is an article in Wikipedia if you are interested about the almogavars. :-)