drivers: led_strip: Add driver for software-based WS2812B operation

This driver uses a bit-banging based technique of generating a signal
for the WS2812B LED strip. Since bit-banging is very timing sensitive,
where each CPU cycle counts, the driver uses inline assembly to
perform the most critical operataions. This initial version of the
driver only supports a Cortex-M0 implementation, and can e.g. be used
with the ZIP Halo LED strip for the BBC microbit:

https://www.kitronik.co.uk/5625-zip-halo-for-the-bbc-microbit.html



Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>