dt-bindings: Add vendor prefix for BSH Hausgeraete GmbH

Document vendor prefix for BSH Hausgeraete GmbH ('BSH Home Appliances',
B/S/H/) manufacturer.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>