drivers: ws2812_gpio: Make timing configurable and less hardware dependend

The current driver contains assembly code which is specific for the nRF51
SOC which makes it incompatible with other SOC's. This patch adds support
for other nRF SOC's as well. The timing is calucated based on the CPU clock
frequency, but can be configured manually as well if needed.

Changes have been verified on a Adafruit Feather nRF52840 Express board,
which contains a single NeoPixel RGB LED. Timings have been verified using
a scope connected to the WS1812 data line.

Signed-off-by: Chaim Zax <chaim.zax@zaxx.pro>