tcl: add SPDX tag

For historical reasons, no license information was added to the
tcl files. This makes trivial adding the SPDX tag through script:
	fgrep -rL SPDX tcl | while read a;do \
	sed -i '1{i# SPDX-License-Identifier: GPL-2.0-or-later\n
	}' $a;done

With no specific license information from the author, let's extend
the OpenOCD project license GPL-2.0-or-later to the files.

Change-Id: Ief3da306a6e1978de7dfb8f552f9ff23151f9944
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7030
Tested-by: jenkins

tcl/mmr_helpers.tcl

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 proc proc_exists { NAME } {
     set n [info commands $NAME]