arm64: dts: msm8916: Add missing #phy-cells

Add a missing #phy-cells to the dsi-phy, to silence dtc warning.

Cc: Archit Taneja <architt@codeaurora.org>
Fixes: 305410ff

 ("arm64: dts: msm8916: Add display support")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>