dt-bindings: display: convert kingdisplay,kd035g6-54nt to DT Schema

v2:
  - Drop use of spi-slave.yaml (Maxime)
  - Introduce unevaluatedProperties (Maxime)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-14-sam@ravnborg.org