ARM: dts: qcom: msm8974: Add compat qcom,msm8974-dsi-ctrl

Add silicon specific compatible qcom,msm8974-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for msm8974 against the yaml documentation.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>