ASoC: dt-bindings: asahi-kasei,ak4619: Add powerdown GPIO

Add documentation for an optional property that enables operation of the
'PDN' (powerdown) pin during initialization:

    codec@10 {
        compatible = "asahi-kasei,ak4619";
        reg = <0x10>;
        powerdown-gpios = <&gpio1 0 GPIO_ACTIVE_LOW>;  // <--
        #sound-dai-cells = <0>;
    };

Signed-off-by: Alexander Koch <mail@alexanderkoch.net>
Link: https://patch.msgid.link/20260929090821.1476257-3-mail@alexanderkoch.net
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/asahi-kasei,ak4619.yaml

@@ -34,6 +34,12 @@ properties:
     $ref: audio-graph-port.yaml#
     unevaluatedProperties: false
 
+  powerdown-gpios:
+    maxItems: 1
+    description:
+      GPIO connected to the 'PDN' pin, if any. Needs to be set low to
+      power down the codec, hence the GPIO must be GPIO_ACTIVE_LOW.
+
 required:
   - compatible
   - reg
@@ -42,6 +48,8 @@ unevaluatedProperties: false
 
 examples:
   - |
+    #include <dt-bindings/gpio/gpio.h>
+
     i2c {
         #address-cells = <1>;
         #size-cells = <0>;
@@ -49,6 +57,8 @@ examples:
             compatible = "asahi-kasei,ak4619";
             reg = <0x10>;
 
+            powerdown-gpios = <&gpio1 0 GPIO_ACTIVE_LOW>;
+
             clocks = <&rcar_sound>;
             clock-names = "mclk";