contrib: replace the BSD-3-Clause license tag

Replace the BSD-3-Clause boilerplate with the SPDX tag.
Add the SPDX tag and the copyright to two makefiles that were
added by TI with the other files in their respective folder.

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: I3ad1b2dbdb6054b74dcc26e394c9223ba0427caf
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7158
Tested-by: jenkins