dt-bindings: Add Keith&Koep vendor prefix

Add vendor prefix for Keith&Koep GmbH , http://keith-koep.com/en/Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Shawn Guo <shawnguo@kernel.org>
[robh: fix alphabetizing]
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -143,6 +143,7 @@ issi	Integrated Silicon Solutions Inc.
 jdi	Japan Display Inc.
 jedec	JEDEC Solid State Technology Association
 karo	Ka-Ro electronics GmbH
+keithkoep	Keith & Koep GmbH
 keymile	Keymile GmbH
 kinetic Kinetic Technologies
 kosagi	Sutajio Ko-Usagi PTE Ltd.