This is the simplest solution for phone charging on a bike.
http://www.revzilla.com/motorcycle/battery-tender-usb-charger
Plugs into your tender lead and just use your usb cable. If you don't have a tender and a lead you really should have one. The Schumacher is a nice unit and can be had at wally mart for around $20-$25
http://www.walmart.com/ip/Schumacher-MC-1-6-12-Volt-1A-Charger-Trickle-6-12-Volt-Each/21642124