dt-bindings: display: Add Himax HX8394 panel controller

Add device tree bindings for panels based on the Himax HX8394 controller,
such as the HannStar HSD060BHW4 720x1440 TFT LCD panel that is connected
through a MIPI-DSI video interface.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102230733.3506624-2-javierm@redhat.com