boards: arm: arduino_nicla_sense_me: Add user button

This commit adds a user button to the board DTS.

The button is not connected to nRESET as suggested
by the board schematics, but rather connected to
P0.21. The source code found in the Arduino IDE
connects an interrupt to this GPIO and calls
NVIC_System_Reset from there to cause the reset.

The button was therefore added as a regular button
to cover the most general case.

This has been verified on an Arduino Nicla Sense ME.

Signed-off-by: Benjamin Björnsson <benjamin.bjornsson@gmail.com>