input: add arduino,modulino-buttons

Add an input driver for the modulino buttons module. This is a pluggable
I2C board implementing three buttons and three LEDs, the I2C protocol is
implemented on a microcontroller on the modulino board itself, the
firmware for that is open source and can be updated using an Arduino
sketch:

Link: https://github.com/arduino/node_modulino_firmware
Link: https://github.com/arduino-libraries/Modulino


Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>