dt-bindings: msm/dp: List supplies in the bindings

We're supposed to list the supplies in the dt bindings but there are
none in the DP controller bindings. Looking at the Linux driver and
existing device trees, we can see that two supplies are expected:
- vdda-0p9-supply
- vdda-1p2-supply

Let's list them both in the bindings. Note that the datasheet for
sc7280 doesn't describe these supplies very verbosely. For the 0p9
supply, for instance, it says "Power for eDP 0.9 V circuits". This
this is obvious from the property name, we don't bother cluttering the
bindings with a description.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sankeerth Billakanti <quic_sbillaka@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/483373/
Link: https://lore.kernel.org/r/20220425140619.1.Ibfde5a26a7182c4b478d570c23d2649823ac2cce@changeid
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>