include/linux/mtd: Update copyright notices

Condense these updates down to SPDX tags too while doing this.  This is
a port of a1452a3771c4eb85bd779790b040efdc36f4274e from the Linux
Kernel.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/linux/mtd/concat.h b/include/linux/mtd/concat.h
index 195a4a5..a374ca9 100644
--- a/include/linux/mtd/concat.h
+++ b/include/linux/mtd/concat.h
@@ -1,9 +1,10 @@
 /*
  * MTD device concatenation layer definitions
  *
- * (C) 2002 Robert Kaiser <rkaiser@sysgo.de>
+ * Copyright © 2002      Robert Kaiser <rkaiser@sysgo.de>
  *
- * This code is GPL
+ * SPDX-License-Identifier:	GPL-2.0+
+ *
  */
 
 #ifndef MTD_CONCAT_H