dt-bindings: display: convert lg,lg4573 to DT Schema

dt-bindings: display: convert lg,lg4573 to DT Schema

v2:
  - Dropped spi-slave (Maxime)
  - Added unevaluatedProperties (Maxime)
  - Deleted needless compatible from example (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-17-sam@ravnborg.org