Fix analog ref enums. (arduino#83)

AR_INTERNAL was removed in arduino#74. Because of this, non-SAMD51 boards fail because AR_INTERNAL isn't defined.
This also define AR_INTERNAL_2V23, which is also used for non-SAMD51
boards.

Author: ademuri

cores/arduino/wiring_analog.c

@@ -228,7 +228,6 @@ void analogReference(eAnalogReference mode)
   syncADC();
   switch (mode)
   {
-    case AR_INTERNAL:
     case AR_INTERNAL2V23:
       ADC->INPUTCTRL.bit.GAIN = ADC_INPUTCTRL_GAIN_1X_Val;      // Gain Factor Selection
       ADC->REFCTRL.bit.REFSEL = ADC_REFCTRL_REFSEL_INTVCC0_Val; // 1/1.48 VDDANA = 1/1.48* 3V3 = 2.2297

cores/arduino/wiring_analog.h

@@ -37,6 +37,7 @@ typedef enum _eAnalogReference
   AR_INTERNAL1V25,
   AR_INTERNAL2V0,
   AR_INTERNAL2V2,
+  AR_INTERNAL2V23,
   AR_INTERNAL2V4,
   AR_INTERNAL2V5,
   AR_INTERNAL1V65,