openocd: fix syntax of SPDX tags

Put the SPDX tag alone in a comment in the first line of the file.
Replace the obsolete GPL-2.0+ 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: Ia91b0f7da42c439b6340bbe81983b86b68f6d65c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7156
Tested-by: jenkins

contrib/loaders/flash/xmc1xxx/erase_check.S

@@ -1,11 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
 /*
  * Infineon XMC1000 flash sector erase check
  *
  * Copyright (c) 2016 Andreas Färber
  *
  * Based on XMC1100 AA-Step Reference Manual
- *
- * License: GPL-2.0+
  */
 
 #include "xmc1xxx.S"