ws2812: add SPI based implementation

[aykevl]

This is useful because it allows using the WS2812 driver on platforms without bitbanged WS2812 support and it may be the only option if disabling interrupts is impossible or impractical.

WIP: mostly untested. A previous version worked on a nrf52832 board. When I tested it on a PyBadge the SPI output looked like garbage (clock worked but data was a mess). Haven't investigated that yet.