If you're not concerned about weatherproofing, just wire up a 12V/Cigarette plug to the bike and then use one of these:
They make a variety of types and sizes. If you use a tankbag, you can run the wire to that and put the 12V outlet in your tankbag. As posted earlier, your bike is making 12V DC. You need an inverter to make it 110AC (typical 110V wall wart) or some type of regulator to make it 5V DC (USB). That Bestek thingy above does both. It's a little bulky, but a pretty simple solution.