Add WS2812B driver (SPI-based)

[salkinium]

This adds a driver for any number of chained WS2812 leds using a 3-bit SPI encoding (0 -> 100, 1 -> 110) running at 3 MHz.

There are several caveats:

1. Since we don't have DMA, this uses the STM32 HAL directly to keep the transmit register full.
2. Atomic lock is not enforced, this should be done externally, if required. (Writing one LED takes 24µs, so this atomic section could become quite long).
3. The memory footprint is 3x as large, due to the bit stuffing for SPI.
4. There is no enforced reset period of at least 50µs after the write is finished, it is up to the user to not trigger another write too early.

Unfortunately due to the SPI HAL being used directly, this driver is STM32 specific.