dt-bindings: display: Add Apple pre-DCP display controller

Add bindings for a secondary display controller present on certain
Apple laptops.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Sasha Finkelstein <fnkl.kernel@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250224-adpdrm-v8-1-cccf96710f0f@gmail.com


Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>