Various Makefiles: Add SPDX-License-Identifier tags

After consulting with some of the SPDX team, the conclusion is that
Makefiles are worth adding SPDX-License-Identifier tags too, and most of
ours have one.  This adds tags to ones that lack them and converts a few
that had full (or in one case, very partial) license blobs into the
equivalent tag.

Cc: Kate Stewart <kstewart@linuxfoundation.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/Makefile b/drivers/Makefile
index ad29a4f..c9031f2 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -1,3 +1,7 @@
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
 obj-$(CONFIG_$(SPL_)DM)		+= core/
 obj-$(CONFIG_$(SPL_)CLK)	+= clk/
 obj-$(CONFIG_$(SPL_)LED)	+= led/