s390: add a few more SPDX identifiers

Add the correct SPDX license to a few more files under arch/s390 and
drivers/s390 which have been missed to far.
The SPDX identifier is a legally binding shorthand, which can be used
instead of the full boiler plate text.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/defkeymap.map

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
 # Default keymap for 3270 (ebcdic codepage 037).
 keymaps 0-1,4-5