Provide constants for analog reference

[matthijskooijman]

The arduino analogReference function allows changing the analog reference voltage used. The Pinoccio is a bit different from other Arduinos, in that it supports 1.8V, 1.6V and 1.5V, but not Vcc. Constants for these voltage should be exposed from pins_arduino.h (but this probably requires some changes to upstream Arduino.h as well, to allow this).