dt-bindings: Add vendor prefix for Netronix, Inc.

Netronix, Inc. (http://www.netronixinc.com/

) makes ebook reader board
designs, which are for example used in Kobo and Tolino devices.

An alternative prefix for Netronix would be "ntx", which is already used
in code released by Netronix. It is shorter, but perhaps less clear.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>