arm64: dts: qcom: sdm660: Add compat qcom,sdm660-dsi-ctrl

Add silicon specific compatible qcom,sdm660-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for sdm660 against the yaml documentation.

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