ASoC: dt-bindings: add compatible string for NAU8318

The audio amplifier NAU8318 is almost functionally identical to NAU8315.
Adds compatible string "nuvoton,nau8318" for driver reuse.
Signed-off-by: David Lin <CTLIN0@nuvoton.com>
Link: https://lore.kernel.org/r/20221124055658.53828-2-CTLIN0@nuvoton.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/nau8315.txt

@@ -2,6 +2,7 @@ Nuvoton NAU8315 Mono Class-D Amplifier
 
 Required properties:
 - compatible   : "nuvoton,nau8315"
+                 "nuvoton,nau8318"
 
 Optional properties:
 - enable-gpios : GPIO specifier for the chip's device enable input(EN) pin.
@@ -16,3 +17,8 @@ nau8315 {
 	compatible = "nuvoton,nau8315";
 	enable-gpios = <&gpio1 5 GPIO_ACTIVE_HIGH>;
 };
+
+nau8318 {
+	compatible = "nuvoton,nau8318";
+	enable-gpios = <&gpio1 5 GPIO_ACTIVE_HIGH>;
+};