Add support for 2 stepper motors

**Is your feature request related to a problem? Please describe.**
Sometimes open-loop control is easier, therefore many projects would like to use stepper motors for wheel control. A simple tank-like robot or other style 2-wheel rover could easily be controlled with 2 stepper motors. 4-wire (bipolar) stepper motors would be easiest via control by 2 motor outputs.

**Describe the solution you'd like**
Ideally, I'd like to hand all the step generation off to the PRUs and have an API that is initialized with 2 motor outputs and then given a number of steps to execute in any direction.

**Describe alternatives you've considered**
A simpler approach could just be a wrapper API for the existing motor drive functions.

**Additional context**
I did a quick proof of concept with BeagleBone Blue and a 4-wire stepper.
https://beagleboard.org/p/jkridner/bipolar-stepper-motor-with-beaglebone-blue-e4950e

I also did a quick hack in JavaScript.
https://gist.github.com/jadonk/7b2d335364b5474d566463d9b45360fa