dt-bindings: display: visionox,rm69299: document reg

Panels are supposed to have one reg.  This fixes dtbs_check warnings
like:

  sc7180-idp.dtb: panel@0: 'reg' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230326155425.91181-5-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>