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 <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

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

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