Add license and copyright to probe_oen.pio

This file is directly attributable to Raspberry Pi.

src/probe_oen.pio

diff --git a/src/probe_oen.pio b/src/probe_oen.pio
index bcf1d29..2057705 100644
--- a/src/probe_oen.pio
+++ b/src/probe_oen.pio
@@ -1,3 +1,6 @@
+; SPDX-License-Identifier: MIT
+; Copyright (c) 2023 Raspberry Pi Ltd
+
 ; Output-enable active-low variant of the SWD probe
 
 ; This program is very similar to the one in probe.pio. The only difference is