dt-bindings: display: convert samsung,s6e63m0 to DT Schema

The binding for this panel is a SPI slave.

v2:
  - Drop use of spi-slave (Maxime)
  - Introude unevaluatedProperties (Maxime)
  - Drop reg entry in example (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: Jonathan Bakker <xc-racer2@live.ca>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-5-sam@ravnborg.org