drivers: led_strip: ws2812_gpio: Add support for nRF52 SOC

[chaim-zax]

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 the nRF52 SOC's as well. The change has minimal impact on existing code to make sure it's fully compatible with the nRF51.

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 WS2812 data line.