drivers: cosmetic: Convert SPDX license tags to Linux Kernel style

Complete in the drivers directory the work started with
commit 83d290c56fab ("SPDX: Convert all of our single
license tags to Linux Kernel style").

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/board/Makefile b/drivers/board/Makefile
index 2224338..c8dab4f 100644
--- a/drivers/board/Makefile
+++ b/drivers/board/Makefile
@@ -1,4 +1,4 @@
-# SPDX-License-Identifier:	GPL-2.0+
+# SPDX-License-Identifier: GPL-2.0+
 #
 # (C) Copyright 2017
 # Mario Six,  Guntermann & Drunck GmbH, mario.six@gdsys.cc