dt-bindings: drm/bridge: ps8640: Add aux-bus child

dp-aux-bus.yaml says we can list an eDP panel as a child of
an eDP controller node to represent the fact that the panel
is connected to the controller's DP AUX bus.

Let's add it to the ps8640 bindings.

Signed-off-by: Philip Chen <philipchen@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929173343.v2.2.I6050ba184b24d887e92692a72ee3054d643d6091@changeid