ynl: re-license uniformly under GPL-2.0 OR BSD-3-Clause

I was intending to make all the Netlink Spec code BSD-3-Clause
to ease the adoption but it appears that:
 - I fumbled the uAPI and used "GPL WITH uAPI note" there
 - it gives people pause as they expect GPL in the kernel
As suggested by Chuck re-license under dual. This gives us benefit
of full BSD freedom while fulfilling the broad "kernel is under GPL"
expectations.

Link: https://lore.kernel.org/all/20230304120108.05dd44c5@kernel.org/
Link: https://lore.kernel.org/r/20230306200457.3903854-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/userspace-api/netlink/specs.rst

@@ -24,6 +24,9 @@ YAML specifications can be found under ``Documentation/netlink/specs/``
 This document describes details of the schema.
 See :doc:`intro-specs` for a practical starting guide.
 
+All specs must be licensed under ``GPL-2.0-only OR BSD-3-Clause``
+to allow for easy adoption in user space code.
+
 Compatibility levels
 ====================