boards: adafruit: add initial support for Macropad RP2040

This commit adds initial support for the Adafruit MacroPad RP2040.

The board is based on the Raspberry Pi RP2040 microcontroller and
includes a 3x4 mechanical keyboard, 12 RGB NeoPixels, a rotary encoder
with push button, a 128x64 OLED display, and a small speaker for audio
feedback.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>