dt-bindings: dsi-controller-main: Document missing msm8976 compatible

When all dsi-ctrl compats were added msm8976 was missed, include it too.

Signed-off-by: Adam Skladowski <a39.skl@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/575288/
Link: https://lore.kernel.org/r/20240121194221.13513-3-a39.skl@gmail.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>