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

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

Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <andersson@kernel.org>
Cc: Konrad Dybcio <konrad.dybcio@somainline.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Jonathan Marek <jonathan@marek.ca>
Cc: Bjorn Andersson <andersson@kernel.org>
Cc: Konrad Dybcio <konrad.dybcio@somainline.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>