dt-bindings: msm: dsi-controller-main: 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. Mark the supplies as required as was previously the case in the
.txt implementation.

Warnings about missing regulators can be resolved by updating the relevant
dtsi files to point to fixed always-on regulators where appropriate.

Fixes: 4dbe55c9 ("dt-bindings: msm: dsi: add yaml schemas for DSI bindings")
Cc: Rob Clark <robdclark@gmail.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>