of: add vendor prefix for Gumstix

Using "gumstix" for Gumstix Inc.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -33,6 +33,7 @@ fsl	Freescale Semiconductor
 GEFanuc	GE Fanuc Intelligent Platforms Embedded Systems, Inc.
 gef	GE Fanuc Intelligent Platforms Embedded Systems, Inc.
 gmt	Global Mixed-mode Technology, Inc.
+gumstix	Gumstix, Inc.
 hisilicon	Hisilicon Limited.
 hp	Hewlett Packard
 ibm	International Business Machines (IBM)