dt-bindings: display: panel: Add binding document for Leadtek LTK050H3146W

The LTK050H3146W is a 5.0" 720x1280 DSI display.

changes in v2:
- add display variants

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200407232351.2547750-1-heiko@sntech.de