dt-bindings: vendor-prefixes: Add an entry for Plymovent

Add "ply" entry for Plymovent Group BV: https://www.plymovent.com/



Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>