tests: spi: Add overlay for frdm_mcxa156 dual test

Add an overlay to the spi_controller_peripheral test for the
frdm_mcxa156. Since this IP has multiple CS, need to introduce a way to
configure the correct CS to the test; just going to use zephyr,user for
now instead of refactoring to be less nordic-specific, keeping the usage
optional.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

tests/drivers/spi/spi_controller_peripheral/boards/frdm_mcxa156.overlay

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2025 NXP
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <lpspi1_header.overlay>
+
+/ {
+	zephyr,user {
+		peripheral-cs = <1>;
+	};
+};
+
+&lpspi0 {
+	dut_spi_dt: test-spi-dev@0 {
+		compatible = "vnd,spi-device";
+		reg = <0>;
+		spi-max-frequency = <10000000>;
+	};
+	transfer-delay = <100>;
+	sck-pcs-delay = <10>;
+	pcs-sck-delay = <10>;
+	/* lower the master interrupt priority so slave can interrupt */
+	interrupts = <28 1>;
+};
+
+dut_spis: &lpspi1 {};

tests/drivers/spi/spi_controller_peripheral/src/main.c

@@ -28,7 +28,8 @@
 static struct spi_dt_spec spim = SPI_DT_SPEC_GET(DT_NODELABEL(dut_spi_dt), SPIM_OP, 0);
 static const struct device *spis_dev = DEVICE_DT_GET(DT_NODELABEL(dut_spis));
 static const struct spi_config spis_config = {
-	.operation = SPIS_OP
+	.operation = SPIS_OP,
+	.slave = DT_PROP_OR(DT_PATH(zephyr_user), peripheral_cs, 0),
 };
 
 static struct k_poll_signal async_sig = K_POLL_SIGNAL_INITIALIZER(async_sig);