Doc: devicetree: bindings: Add vendor prefix entry - lwn

This patch adds entry for LWN - the Liebherr-Werk Nenzing GmbH company to
vendor-prefixes.txt file.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>