dt-bindings: display/msm/mdp4: require 4 IOMMUs

dt-bindings: display/msm/mdp4: require 4 IOMMUs

APQ8064, the only supported mdp4 platform, uses 4 IOMMUs for the MDP4
device. Update schema accordingly. When we have other MDP4 platforms,
this spec can be updated accordingly.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/492541/
Link: https://lore.kernel.org/r/20220706145222.1565238-5-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>