dt-bindings: add vendor prefix for Next Thing Co.

Next Thing Co. is the company behind the C.H.I.P. and C.H.I.P. Pro
miniature single board computers.
The "nextthing" vendor-prefix is already used for these two board as
well as their own "GR8" SoC.

Cc: Alexander Kaplan <alex@nextthing.co>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Rob Herring <robh@kernel.org>