arm64: dts: qcom: sc7180: Add compat qcom,sc7180-dsi-ctrl

Add silicon specific compatible qcom,sc7180-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for sc7180 against the yaml documentation.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>