boards: adafruit: Add support for Adafruit Feather RP2040.

Added support for the Adafruit Feather RP2040, based on existing board
definitions for the Qt Py RP2040 and others.

Board image (c) Kattni Rembor, CC-BY-SA-3.0 per licensing info at
https://learn.adafruit.com/assets/100345

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Signed-off-by: Anson Mansfield <amansfield@mantaro.com>