dt-bindings: drm/bridge: anx7625: Add power supplies

anx7625 requires 3 power supply regulators.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401053202.159302-1-hsinyi@chromium.org