cores: arduino: zephyrCommon: make analogReference as weak func

Make analogReference as weak to allow override by each board
implimentations.

Co-Authored-by: Martino Facchin <m.facchin@arduino.cc>
Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: facchinm

cores/arduino/zephyrCommon.cpp

@@ -457,7 +457,7 @@ void analogWrite(enum dacPins dacName, int value) {
 
 #ifdef CONFIG_ADC
 
-void analogReference(uint8_t mode) {
+void __attribute__((weak)) analogReference(uint8_t mode) {
 	/*
 	 * The Arduino API not clearly defined what means of
 	 * the mode argument of analogReference().