Update P9 to be UNUSED on V2 (was shared on V1)

P9 is a spare pin on micro:bit V2, but was shared with the display on V1

Full pinout is available https://tech.microbit.org/hardware/edgeconnector/

See #57

Author: jaustin

src/codal_port/microbit_pin.c

@@ -37,7 +37,7 @@ const microbit_pin_obj_t microbit_p5_obj  = {{&microbit_dig_pin_type},   5, MICR
 const microbit_pin_obj_t microbit_p6_obj  = {{&microbit_dig_pin_type},   6, MICROBIT_HAL_PIN_P6,  MODE_DISPLAY};
 const microbit_pin_obj_t microbit_p7_obj  = {{&microbit_dig_pin_type},   7, MICROBIT_HAL_PIN_P7,  MODE_DISPLAY};
 const microbit_pin_obj_t microbit_p8_obj  = {{&microbit_dig_pin_type},   8, MICROBIT_HAL_PIN_P8,  MODE_UNUSED};
-const microbit_pin_obj_t microbit_p9_obj  = {{&microbit_dig_pin_type},   9, MICROBIT_HAL_PIN_P9,  MODE_DISPLAY};
+const microbit_pin_obj_t microbit_p9_obj  = {{&microbit_dig_pin_type},   9, MICROBIT_HAL_PIN_P9,  MODE_UNUSED};
 const microbit_pin_obj_t microbit_p10_obj = {{&microbit_ad_pin_type},   10, MICROBIT_HAL_PIN_P10, MODE_DISPLAY};
 const microbit_pin_obj_t microbit_p11_obj = {{&microbit_dig_pin_type},  11, MICROBIT_HAL_PIN_P11, MODE_BUTTON};
 const microbit_pin_obj_t microbit_p12_obj = {{&microbit_dig_pin_type},  12, MICROBIT_HAL_PIN_P12, MODE_UNUSED};