mxssb: Update copyright and description

It's year 2013 now, update the copyright message. Since the tool
now also supports MX233, mention it in the description.
Signed-off-by: Marek Vasut <marex@denx.de>

mxssb.c

 /*
- * Freescale i.MX28 SB image generator
+ * Freescale i.MX23/i.MX28 SB image generator
  *
- * Copyright (C) 2012 Marek Vasut <marex@denx.de>
+ * Copyright (C) 2012-2013 Marek Vasut <marex@denx.de>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by