devicetree: Add vendor-prefix for DH electronics GmbH

Add vendor prefix for DH electronics GmbH, https://www.dh-electronics.com

 .
The company is a SoM and evaluation board manufacturer.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>