tty: fix up a few remaining files without SPDX identifiers

There were a few straggling files under drivers/tty/ that did not have
any SPDX identifier either because they entered the tree recently, or
they somehow missed the mass-tagging of commit b24413180f ("License
cleanup: add SPDX GPL-2.0 license identifier to files with no license")

This commit follows the same rule as b24413180f ("License cleanup: add
SPDX GPL-2.0 license identifier to files with no license") where files
without any specified license in them fall under GPL-2.0 as the correct
license for the individual file.  Add that identifier to these remaining
files so that we don't have to guess at the license of them in the
future.

Cc: Jiri Slaby <jslaby@suse.com>
Cc: "David S. Miller" <davem@davemloft.net>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sn_console.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 /*
  * C-Brick Serial Port (and console) driver for SGI Altix machines.
  *