drivers: spi: siwx91x: spi_loopback test support

1. Add board config and overlay files
2. Update testcase.yaml

Signed-off-by: Sai Santhosh Malae <Santhosh.Malae@silabs.com>

Author: smalae

tests/drivers/spi/spi_loopback/boards/siwx917_rb4338a.conf

@@ -0,0 +1 @@
+CONFIG_SPI_SILABS_SIWX91X_GSPI_DMA=y

tests/drivers/spi/spi_loopback/boards/siwx917_rb4338a.overlay

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2025 Silicon Laboratories Inc.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&pinctrl0 {
+	spi0_default: spi0_default {
+		out {
+			pinmux = <GSPI_CLK_PB9>, <GSPI_MOSI_PB11>;
+		};
+		in {
+			pinmux = <GSPI_MISO_PB10>;
+		};
+	};
+};
+
+&spi0 {
+	status = "okay";
+	cs-gpios = <&gpiob 12 GPIO_ACTIVE_LOW>;
+	pinctrl-0 = <&spi0_default>;
+	pinctrl-names = "default";
+
+	dmas =  <&dma0 11>, <&dma0 10>;
+	dma-names = "tx", "rx";
+
+	slow@0 {
+		compatible = "test-spi-loopback-slow";
+		reg = <0>;
+		spi-max-frequency = <500000>;
+	};
+	fast@1 {
+		compatible = "test-spi-loopback-fast";
+		reg = <1>;
+		spi-max-frequency = <10000000>;
+	};
+};
+
+&dma0 {
+	status = "okay";
+};

tests/drivers/spi/spi_loopback/testcase.yaml

@@ -305,3 +305,7 @@ tests:
     extra_configs:
       - CONFIG_SPI_SILABS_EUSART_DMA=y
       - CONFIG_SPI_ASYNC=n
+  drivers.spi.silabs_siwx91x.loopback:
+    filter: CONFIG_SOC_FAMILY_SILABS_SIWX91X
+    platform_allow:
+      - siwx917_rb4338a