dt-bindings: display: panel: Add binding document for Leadtek LTK500HD1829

The LTK500HD1829 is a 5.0" 720x1280 DSI display.

v3:
  - fixed example by adding address,size-cells (sam)

changes in v2:
- fix id (Maxime)
- drop port (Maxime)

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224112641.30647-2-heiko@sntech.de