dt-bindings/display/panel: otm8009a: Add optional power-supply property

Some boards use a dedicated voltage regulator for this panel.
Add & document this related optional power-supply property.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205094532.23547-2-philippe.cornu@st.com