Add _gpio() function to document magic numbers
Add _gpio() function to document magic numbers

dominicgs/GreatFET-experimental,greatscottgadgets/greatfet,greatscottgadgets/greatfet,greatscottgadgets/greatfet,greatscottgadgets/greatfet,dominicgs/GreatFET-experimental,dominicgs/GreatFET-experimental