dt-bindings: msm: dsi: Add vdd* descriptions back in

When converting from .txt to .yaml we didn't include descriptions for the
existing regulator supplies.

- vdd
- vdda
- vddio

Add those descriptions into the yaml now as they were prior to the
conversion.

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