ARM: dts: qcom: apq8064: add compat qcom,apq8064-dsi-ctrl

Append silicon specific compatible qcom,apq8064-dsi-ctrl to the
mdss-dsi-ctrl block. This allows us to differentiate the specific bindings
for apq8064 against the yaml documentation.

Reviewed-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>