Add board.LED for Grand Central M4 Express

Fixes #5111

Author: NathanY3G

ports/atmel-samd/boards/grandcentral_m4_express/pins.c

@@ -137,6 +137,7 @@ STATIC const mp_rom_map_elem_t board_global_dict_table[] = {
 
     { MP_OBJ_NEW_QSTR(MP_QSTR_NEOPIXEL), MP_ROM_PTR(&pin_PC24) },
 
+    { MP_OBJ_NEW_QSTR(MP_QSTR_LED), MP_ROM_PTR(&pin_PB01) },
     { MP_OBJ_NEW_QSTR(MP_QSTR_LED_RX), MP_ROM_PTR(&pin_PC31) },
     { MP_OBJ_NEW_QSTR(MP_QSTR_LED_TX), MP_ROM_PTR(&pin_PC30) },
     { MP_ROM_QSTR(MP_QSTR_I2C), MP_ROM_PTR(&board_i2c_obj) },