dt: bindings: add vendor prefix for Kamstrup A/S

Kamstrup manufactures meters for electricity, heating, cooling
and water. Including long-life communication infrastructure
for e.g. smart grid based on Linux, more information on
https://www.kamstrup.com



Signed-off-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>