Add a LICENSING file with the Cisco U-Boot GPLv2 exception

[aidangarske]

Context

wolfSPDM is compiled into libwolftpm once wolfTPM switches to the submodule. wolfSSL and wolfTPM carry the Cisco U-Boot GPLv2 exception; wolfSPDM needs the same so the combined library keeps the same terms.

Origin

main...wolftpm-core

What is needed

• Add LICENSING to wolfSPDM with the same exception text wolfTPM uses.
• Ship it in make dist and in wolfTPM's dist (EXTRA_DIST).

Blockers and dependencies

Confirmation of the exact exception text from wolfSSL licensing.

Acceptance criteria

wolfSPDM and wolfTPM dist tarballs both contain the exception text.