boards: arm: adafruit_feather_nrf52840: add voltage divider for battery

This allows firmware to read the voltage for LiPoly battery.

Schematic:
https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/downloads



Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>