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/mailbox/Makefile b/drivers/mailbox/Makefile
index 243ff6f..cd23769 100644
--- a/drivers/mailbox/Makefile
+++ b/drivers/mailbox/Makefile
@@ -1,6 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+#
 # Copyright (c) 2016, NVIDIA CORPORATION.
 #
-# SPDX-License-Identifier: GPL-2.0
 
 obj-$(CONFIG_$(SPL_)DM_MAILBOX) += mailbox-uclass.o
 obj-$(CONFIG_SANDBOX_MBOX) += sandbox-mbox.o