The .txt binding explains:

"
    The following optional properties only apply to
    RGB and YUV input modes and
    can be omitted for BT.656 input modes:
"

This constraint is not implmented in the DT Schema.

The original binding from the .txt file referenced
properties that is included in panel-timing.yaml.

The properties in question are:
  - pixelclk-active
  - de-active
  - hsync-active
  - vsync-active

These properties was dropped in the conversion as they are not relevant.

v2:
  - drop properties from panel-timing (Linus)
  - drop use of spi-slave.yaml (Maxime)
  - introduce unevaluatedProperties (Maxime)
  - dropped unused properties (Linus)
  - delete stray spaces
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-8-sam@ravnborg.org