Fix the SPDX-License-Identifier in DMC-620 files

The SPDX-License-Identifier was wrong in the Arm CoreLink DMC-620
driver files. It used the incorrect FreeBSD variant of the BSD-2-Clause
identifier. According to [1] all files should use BSD-2-Clause.

[1] https://tools.spdx.org/app/check_license/

Reported by: emaste
Sponsored by: Arm Ltd