treewide: Add SPDX license identifier - Makefile/Kconfig

Add SPDX license identifiers to all Make/Kconfig files which:

 - Have no license information of any form

These files fall under the project license, GPL v2 only. The resulting SPDX
license identifier is:

  GPL-2.0-only

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/mxsfb/Kconfig

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 config DRM_MXS
 	bool
 	help

drivers/gpu/drm/mxsfb/Makefile

@@ -1,2 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
 mxsfb-y := mxsfb_drv.o mxsfb_crtc.o mxsfb_out.o
 obj-$(CONFIG_DRM_MXSFB)	+= mxsfb.o