dt-bindings: display: msm: Rename mdss node name in example

Follow other YAMLs and replace mdss name into display-subystem.

Signed-off-by: Adam Skladowski <a39.skl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Fixes: b93bdff4 ("dt-bindings: display/msm: add support for SM6115")
Fixes: 06097b13

 ("dt-bindings: display/msm: split dpu-qcm2290 into DPU and MDSS parts")
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/513585/
Link: https://lore.kernel.org/r/20221130200950.144618-2-a39.skl@gmail.com
Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>