dts: bindings: fix label properties for gpio-leds node

This patch adds a label property to the GPIO LED parent node and fixes
the label property description of the child node. Indeed it is the label
of a parent node which can be used as device_get_binding() parameter to
retrieve the LED device, and not the one of a child node.

In addition, this patch sets both this properties as not required
because they are not.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>

dts/bindings/gpio/gpio-leds.yaml

@@ -5,13 +5,28 @@ description: GPIO LEDs parent node
 
 compatible: "gpio-leds"
 
+include:
+    - name: base.yaml
+      property-allowlist: [label]
+
+properties:
+    label:
+      description: |
+        Human readable string describing the device and used to set the device
+        name. It can be passed as argument to device_get_binding() to retrieve
+        the device. If this property is omitted, then the device name is set
+        from the node full name.
+
 child-binding:
     description: GPIO LED child node
     properties:
        gpios:
           type: phandle-array
           required: true
        label:
-          required: true
+          required: false
           type: string
-          description: Human readable string describing the device (used as device_get_binding() argument)
+          description: |
+            Human readable string describing the LED. It can be used by an
+            application to identify this LED or to retrieve its number/index
+            (i.e. child node number) on the parent device.