boards: arm: Add adafruit_itsybitsy_nrf52840 board

Adds support for the Adafruit ItsyBitsy nRF52840 Express.

This board ships with the UF2 bootloader, so the device has
been set up so that Zephyr applications are flashed the
same way that other (Adafruit) firmware is flashed with the
UF2 bootloader.

This has been tested locally, and the button, blinky and
led_apa102 samples run without problems.

Signed-off-by: Embla Flatlandsmo <embla.flatlandsmo@gmail.com>
12 files changed