openocd: src/flash: replace the GPL-2.0-or-later license tag

openocd: src/flash: replace the GPL-2.0-or-later license tag

Replace the FSF 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: Ic7db91fe37d1139d42c99e303b3243b6c8fe3ea2
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7067
Tested-by: jenkins