dt-bindings: display: convert samsung,ld9040 to DT Schema

v2:
  - drop use of spi-slave.yaml (Maxime)
  - added unevaluatedProperties (Maxime)
  - added type to width/height properties (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-23-sam@ravnborg.org