dt-bindings: msm: dsi-phy-28nm: Add missing qcom,dsi-phy-regulator-ldo-mode

Add in missing qcom,dsi-phy-regulator-ldo-mode to the 28nm DSI PHY.
When converting from .txt to .yaml we missed this one.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>