arm64: dts: qcom: msm8996: add missing DSI clock assignments

Add missing DSI clock assignments to properly use DSI PHY clocks as DSI
byte and pixel clock parents.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220706145412.1566011-1-dmitry.baryshkov@linaro.org