openocd: src: replace the GPL and BSD-Source-Code license tags

openocd: src: replace the GPL and BSD-Source-Code license tags

Add the new license text in the license pool.
Replace the GPL and BSD boilerplates with the SPDX tag.
Add the copyright owner of Atmel, as it was explicitly listed in
the BSD boilerplate text.

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: Ibb117dbf8402269be3e5ba4f4c472162494d813f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7062
Tested-by: jenkins