openocd: src: replace the BSD-3-Clause license tag

openocd: src: replace the BSD-3-Clause license tag

Replace the BSD boilerplate with the SPDX tag.

The SPDX tag on files *.c is incorrect, as it should use the C99
single line comment using '//'. But current checkpatch doesn't
allow C99 comments, so keep using standard C comments, by now.

Change-Id: I5a8cab2051eee7eb99adf67f9631b0827c1359de
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7059
Tested-by: jenkins