dt-bindings: gpio: modepin: Describe label property

Describe optional label property which can be used for better gpio
identification.

Signed-off-by: Michal Simek <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/devicetree/bindings/gpio/xlnx,zynqmp-gpio-modepin.yaml

···
   "#gpio-cells":
     const: 2
 
+  label: true
+
 required:
   - compatible
   - gpio-controller
···
             compatible = "xlnx,zynqmp-gpio-modepin";
             gpio-controller;
             #gpio-cells = <2>;
+            label = "modepin";
         };
     };