add missing AREF pin

Author: ladyada

ports/atmel-samd/boards/feather_m0_express/pins.c

@@ -1,6 +1,7 @@
 #include "shared-bindings/board/__init__.h"
 
 STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_AREF), MP_ROM_PTR(&pin_PA03) },
     { MP_ROM_QSTR(MP_QSTR_A0), MP_ROM_PTR(&pin_PA02) },
     { MP_ROM_QSTR(MP_QSTR_A1), MP_ROM_PTR(&pin_PB08) },
     { MP_ROM_QSTR(MP_QSTR_A2), MP_ROM_PTR(&pin_PB09) },