Add WS2812B LED driver (Neopixel's)

This driver presents a /dev/ws2812 interface to which you write
a buffer containing RGB values (LSByte is red, MSByte is unused).
This driver requires changes to the pin setup to move PWM to an
accessible pin (by default PWM comes out of the audio), this can
be done by adding dt-blob.bin into /boot.  See RaspberryPi
documentation for information about configuration and the dt-blob.

Also requires changes to PWM frequency to be 2.4MHz