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

Documentation/devicetree/bindings/display/panel/sitronix,st7789v.yaml

@@ -26,6 +26,10 @@ properties:
   spi-cpha: true
   spi-cpol: true
 
+  dc-gpios:
+    maxItems: 1
+    description: DCX pin, Display data/command selection pin in parallel interface
+
 required:
   - compatible
   - reg