The SAMD21 implementation is an adaption of @jimmo's code for STM32Lxx.
The only changes are the addresses and names of the port registers and the
timing parameters.
SAMD21: The precision is about +/-25ns at 48MHz clock frequency. The first
two cycles are about 40-60 ns longer than set. But still good enough to
drive a neopixel device.
SAMD51: The precision is about +/-30ns at 120MHz clock frequency. Good
enough to drive a neopixel device.
