arm64: dts: qcom: sm8450: Add compat qcom,sm8450-dsi-ctrl

Add silicon specific compatible qcom,sm8450-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for sm8450 against the yaml documentation.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230110055433.734188-3-dmitry.baryshkov@linaro.org