legal: add spdx header to serial_decode_zpc.py

The project ZGW is reusing the script serial_decode_zpc.py. Adding the
SPDX headers to have a clear status on the license and copyright of the
script across projects.

Author: lmolina

scripts/serial_decode_zpc.py

@@ -1,4 +1,6 @@
 #!/usr/bin/python
+# SPDX-License-Identifier: LicenseRef-MSLA
+# SPDX-FileCopyrightText: Silicon Laboratories Inc. https://www.silabs.com
 '''
 Created on 08/11/2011