dt-bindings: display: msm: Make "additionalProperties: true" explicit

dt-bindings: display: msm: Make "additionalProperties: true" explicit

Make it explicit that child nodes have additional properties and the
child node schema is not complete. The complete schemas are applied
separately based the compatible strings.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Patchwork: https://patchwork.freedesktop.org/patch/559387/
Link: https://lore.kernel.org/r/20230925212434.1972368-2-robh@kernel.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>