boards: arm: particle_*: add label for arduino_i2c

Zephyr shields are mostly defined using references to the arduino
headers.  There are featherwing shields like the Adafruit SSD1306
128x32 display that could work on feather form-factor devices if the
arduino I2C label was available.  Add that label.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>