arm64: dts: qcom: sm8250: Add compat qcom,sm8250-dsi-ctrl

Add silicon specific compatible qcom,sm8250-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for sm8250 against the yaml documentation.

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