dt-bindings: msm: dsi-phy-28nm: Document missing #phy-cells dependency

We are missing #phy-cells in this binding. Add it in as a dependency with
a const value of 0 covering all current usages of this phy.

arch/arm/boot/dts/qcom-msm8974.dtsi
arch/arm/boot/dts/qcom-apq8064.dtsi
arch/arm64/boot/dts/qcom/msm8916.dtsi

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