samples: i2s: output: add nucleo_g431kb

Add nucleo_g431kb board in samples/drivers/i2s/output

Signed-off-by: Mario Paja <mariopaja@hotmail.com>

Author: mariopaja

samples/drivers/i2s/output/boards/nucleo_g431kb.conf

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+CONFIG_HEAP_MEM_POOL_SIZE=4192

samples/drivers/i2s/output/boards/nucleo_g431kb.overlay

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+/*
+ * Copyright (c) 2025 Mario Paja <mariopaja@hotmail.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	aliases {
+		i2s-tx = &sai1_a;
+	};
+};
+
+&pll {
+	/* 44.1KHz (-0.03% Error) */
+	div-q = <8>;
+};
+
+&sai1_a {
+	pinctrl-0 = <&sai1_sd_a_pa10 &sai1_fs_a_pa9
+		&sai1_sck_a_pa8>;
+	pinctrl-names = "default";
+	status = "okay";
+	dma-names = "tx";
+};
+
+&dma1{
+	status = "okay";
+};
+
+&dmamux1{
+	status = "okay";
+};