stepper-pinmux demo

@beaglebone:~$ show-pins | sort | tail -n4
P9.41                            106 fast rx down 7 gpio 3.20        stepper-pinmux (stepper-disabled)
P9.41 / jtag emu3                109 fast rx      7 gpio 0.20        stepper-pinmux (stepper-unused)
P9.42                             89 fast rx      7 gpio 0.07        stepper-pinmux (stepper-unused)
P9.42                            104 fast rx down 7 gpio 3.18        stepper-pinmux (stepper-disabled)
@beaglebone:~$ python3 -c 'from bone_pinmux import *; set_pinmux_state("stepper", "active")'
@beaglebone:~$ show-pins | sort | tail -n4
P9.41                            106 fast rx down 5 pru 0 out 6      stepper-pinmux (stepper-active)
P9.41 / jtag emu3                109 fast rx      7 gpio 0.20        stepper-pinmux (stepper-unused)
P9.42                             89 fast rx      7 gpio 0.07        stepper-pinmux (stepper-unused)
P9.42                            104 fast rx down 5 pru 0 out 4      stepper-pinmux (stepper-active)