dt-bindings: display: sitronix,st7789v: document dc-gpios

The device comes with DCX pin which is already used in
canaan/sipeed_maixduino.dts (although not in Linux driver).

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230326164700.104570-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>