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>