x86: Set the SMP flag when MP init is complete

Set this flag so we can track when it is safe to use CPUs other than the
main one.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/mp_init.c

@@ -643,6 +643,7 @@ int mp_init(void)
 		debug("CPU init failed: err=%d\n", ret);
 		return ret;
 	}
+	gd->flags |= GD_FLG_SMP_READY;
 
 	return 0;
 }