renesas-ra: Use MP_REGISTER_ROOT_POINTER().

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

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

ports/renesas-ra/mphalport.c

@@ -118,3 +118,5 @@ void mp_hal_get_mac_ascii(int idx, size_t chr_off, size_t chr_len, char *dest) {
         *dest++ = hexchr[mac[chr_off >> 1] >> (4 * (1 - (chr_off & 1))) & 0xf];
     }
 }
+
+MP_REGISTER_ROOT_POINTER(struct _machine_uart_obj_t *pyb_stdio_uart);