dt-bindings: vendor-prefixes: add mntre

MNT Research GmbH develops open source hardware, software
and textiles in Berlin, Germany.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>