Add support for blackmagic on feather-nrf52840

[azlekov]

I can confirm that the feather is working pretty well with the blackmagic probe. Before the new zephyr version which support the feather out of the box I was using the DK with blackmagic and everything works, except the wrong dt mapping of course.

Previous working configuration for the feather with blackmagic:

[env:nrf52840_dk]
platform = nordicnrf52
board = nrf52840_dk
framework = zephyr
...