Use PRU/ADC driver to capture encoder ticks

@Rowland,

Do not merge this, but please look at https://github.com/pgmmpk/beaglebone_pru_adc - it provides a simpler (better?) way of capturing ADC inputs and encoders.

As to my clone of quickbot_bbb, its still very much work-in-progress. ENCVAL does work and seem to return reasonable values. ENCVEL is completely untested yet, IR readings not tested either.