spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

linuxkpi: Add non-atomic readq()/writeq()

The non-atomic versions are required by drm-510-kmod to build on 32-bit
architectures.

Approved by: hselasky, manu, tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36507