dt-bindings: display: simple: hardware can use ddc-i2c-bus

Both hardware and driver can communicate DDC over i2c bus.

Fixes warnings as:
arch/arm/boot/dts/tegra20-paz00.dt.yaml: panel: 'ddc-i2c-bus' does not match any of the regexes: 'pinctrl-[0-9]+'
	From schema: /home/runner/work/linux/linux/Documentation/devicetree/bindings/display/panel/panel-simple.yaml

Signed-off-by: David Heidelberg <david@ixit.cz>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210927214503.36012-1-david@ixit.cz