Add dummy analogReference() for compatibility

Reference:
https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

Same as Arduino SAM Boards (Due)

Fix #403

Signed-off-by: Frederic.Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/wiring_analog.c

@@ -59,6 +59,10 @@ static inline uint32_t mapResolution(uint32_t value, uint32_t from, uint32_t to)
   }
 }
 
+void analogReference(eAnalogReference ulMode)
+{
+  UNUSED(ulMode);
+}
 
 //perform the read operation on the selected analog pin.
 //the initialization of the analog PIN is done through this function

cores/arduino/wiring_analog.h

@@ -25,6 +25,7 @@ extern "C" {
 
 /*
  * \brief SAM3 products have only one reference for ADC
+ * This is kept only for compatibility with existing AVR based API.
  */
 typedef enum _eAnalogReference {
   AR_DEFAULT,