Stripped-down signal loop for timing purposes

Implements a simple counter-based signal loop on the PRU which doesn't actually
use the data copied in by the host, rather sends a fixed number of ones or
zeros.

For this I've found that a timing of T0H = 350ns and T1H = 750ns works well, as
specified in the WS2812 datasheet, except that a period of 2500ns appears to be
required (contrary to the datasheet).
QBLT trouble
Use PASM struct for counters and pointers
Fix increment error
Load count from host data
INC macro
Working duty cycle modulation?
RGB test pattern