dt-bindings: Add vendor prefix for LeMaker

This introduces a new device-tree binding vendor prefix for Shenzhen
LeMaker Technology Co., Ltd.

This vendor was already in use but it was not documented until now.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Rob Hering <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181107181843.27628-5-contact@paulk.fr