dt-bindings: display: convert ilitek,ili9881c to DT Schema

Updating this binding identified an issue in the example in
the allwinner,sun6i-a31-mipi-dsi binding.
Fix the example so no new warnings are introduced.

v2:
  - fix example in allwinner,sun6i-a31-mipi-dsi (Rob)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: 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/20200408195109.32692-9-sam@ravnborg.org