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/mtd.h b/include/linux/mtd/mtd.h
index c2cd3df..9da77ec 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -1,7 +1,7 @@
 /*
  * Copyright © 1999-2010 David Woodhouse <dwmw2@infradead.org> et al.
  *
- * Released under GPL
+ * SPDX-License-Identifier:	GPL-2.0+
  *
  */