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.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221223021025.1646636-10-bryan.odonoghue@linaro.org