treewide: Add SPDX identifier to IETF ASN.1 modules

Per section 4.c. of the IETF Trust Legal Provisions, "Code Components"
in IETF Documents are licensed on the terms of the BSD-3-Clause license:

https://trustee.ietf.org/documents/trust-legal-provisions/tlp-5/

The term "Code Components" specifically includes ASN.1 modules:

https://trustee.ietf.org/documents/trust-legal-provisions/code-components-list-3/



Add an SPDX identifier as well as a copyright notice pursuant to section
6.d. of the Trust Legal Provisions to all ASN.1 modules in the tree
which are derived from IETF Documents.

Section 4.d. of the Trust Legal Provisions requests that each Code
Component identify the RFC from which it is taken, so link that RFC
in every ASN.1 module.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>