dt-bindings: display: Document the EDT et* displays in one file.

Document the Emerging Display Technology Corp. (EDT) using the
simple-panel binding in one single file.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jan Tuerk <jan.tuerk@emtrion.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180619095546.24445-1-jan.tuerk@emtrion.com