boards: adafruit: add initial support for feather m4 express

The Adafruit Feather M4 Express is a compact, lightweight
ARM development board with an onboard mini NeoPixel, 2 MiB
of SPI flash, charging status indicator and user LEDs, USB
connector, 21 GPIO pins and a small prototyping area.

Signed-off-by: Teresa Zepeda Ventura <teresa.zvent@gmail.com>