dt-bindings: display: panel-simple-dsi: allow vddio variant

Few panels like Samsung s6e3fc2x01 and sofef00 use vddio-supply instead
of power-supply (in DTS and Linux driver), so allow it to fix:

  sdm845-oneplus-enchilada.dtb: panel@0: 'power-supply' is a required property

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230326155425.91181-3-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>