dt-bindings: display: convert sitronix,st7789v to DT Schema

v2:
    - dropped use of spi-slave.yaml (Maxime)
    - added unevaluatedProperties (Maxime)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-28-sam@ravnborg.org