esp8266: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register all port-specific root
pointers in the esp2866 port.

Signed-off-by: David Lechner <david@pybricks.com>

ports/esp8266/machine_uart.c

@@ -329,3 +329,5 @@ const mp_obj_type_t pyb_uart_type = {
     .protocol = &uart_stream_p,
     .locals_dict = (mp_obj_dict_t *)&pyb_uart_locals_dict,
 };
+
+MP_REGISTER_ROOT_POINTER(byte * uart0_rxbuf);