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>

drivers/soc/Makefile

@@ -1,5 +1,7 @@
 #
 # Makefile for the U-boot SOC specific device drivers.
 #
+# SPDX-License-Identifier:	GPL-2.0+
+#
 
 obj-$(CONFIG_ARCH_KEYSTONE)	+= keystone/

drivers/soc/keystone/Makefile

@@ -1 +1,5 @@
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
 obj-$(CONFIG_TI_KEYSTONE_SERDES) += keystone_serdes.o