Use VDD (5V) as default analog reference

Replicates the behaviour on a UNO

Author: facchinm

cores/arduino/wiring.c

@@ -347,6 +347,7 @@ void init()
 
 	/* Enable ADC */
 	ADC0.CTRLA |= ADC_ENABLE_bm;
+	analogReference(VDD);
 
 #endif