That is an easy problem to solve. There are a slew little transformers to can get. I am considering an accessory for this that has an externally (and semi-permanently) mounted GPS antenna/charger/remote that will attach to the left bar so you can control the app with you left hand still on the bars and get superior realtime GPS info while you are riding. Plus, you can then make/buy a weather proof case for your phone and still have full access to the app.
The remote control will be built with something like this
with something like this
stick it in a case with controls something like this