ASoC: dt-bindings: maxim,max9759: Convert to DT schema.

Convert Maxim MAX9759 Speaker Amplifier bindings to DT schema.
Add missing gpio header file into examples section
Signed-off-by: Otabek Nazrullaev <otabeknazrullaev1998@gmail.com>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230315173404.7215-1-otabeknazrullaev1998@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/maxim,max9759.txt

-Maxim MAX9759 Speaker Amplifier
-===============================
-
-Required properties:
-- compatible : "maxim,max9759"
-- shutdown-gpios : the gpio connected to the shutdown pin
-- mute-gpios : the gpio connected to the mute pin
-- gain-gpios : the 2 gpios connected to the g1 and g2 pins
-
-Example:
-
-max9759: analog-amplifier {
-	compatible = "maxim,max9759";
-	shutdown-gpios = <&gpio3 20 GPIO_ACTIVE_LOW>;
-	mute-gpios = <&gpio3 19 GPIO_ACTIVE_LOW>;
-	gain-gpios = <&gpio3 23 GPIO_ACTIVE_LOW>,
-		     <&gpio3 25 GPIO_ACTIVE_LOW>;
-};

Documentation/devicetree/bindings/sound/maxim,max9759.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/sound/maxim,max9759.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Maxim MAX9759 Speaker Amplifier
+
+maintainers:
+  - Otabek Nazrullaev <otabeknazrullaev1998@gmail.com>
+
+properties:
+  compatible:
+    const: maxim,max9759
+
+  shutdown-gpios:
+    maxItems: 1
+    description: the gpio connected to the shutdown pin
+
+  mute-gpios:
+    maxItems: 1
+    description: the gpio connected to the mute pin
+
+  gain-gpios:
+    maxItems: 2
+    description: the 2 gpios connected to the g1 and g2 pins
+
+required:
+  - compatible
+  - shutdown-gpios
+  - mute-gpios
+  - gain-gpios
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+    amplifier {
+      compatible = "maxim,max9759";
+      shutdown-gpios = <&gpio3 20 GPIO_ACTIVE_LOW>;
+      mute-gpios = <&gpio3 19 GPIO_ACTIVE_LOW>;
+      gain-gpios = <&gpio3 23 GPIO_ACTIVE_LOW>,
+             <&gpio3 25 GPIO_ACTIVE_LOW>;
+    };