I found a datasheet for the AR9331. The specs on the GPIO are lacking detail, but I did find the following: VOH = 2.44 V VOL = 0.1 V VIH = 0.7 V VIL = 0.3 V (These are min/max values as appropriate.) I'm not finding a maximum voltage for the GPIO pins. The datasheet I found is a preliminary from December 2010. I'm hoping a newer datasheet is available that has more complete electrical info.