samples: cellular: modem_shell: Add nRF9251 DK support

Added board overlays for the nRF9251 DK.

Signed-off-by: Tommi Kangas <tommi.kangas@nordicsemi.no>

Author: tokangas

samples/cellular/modem_shell/boards/nrf9251dk_nrf9251_cpuapp.conf

@@ -0,0 +1,9 @@
+#
+# Copyright (c) 2026 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+
+# FOTA not yet supported
+CONFIG_MOSH_FOTA=n
+CONFIG_FOTA_DOWNLOAD=n

samples/cellular/modem_shell/boards/nrf9251dk_nrf9251_cpuapp.overlay

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2026 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+/delete-node/ &cpuapp_boot_partition;
+/delete-node/ &cpuapp_slot0_partition;
+/delete-node/ &cpuapp_slot1_partition;
+/delete-node/ &cpuppr_code_partition;
+/delete-node/ &cpuflpr_code_partition;
+/delete-node/ &storage_partition;
+/delete-node/ &periphconf_partition;
+/delete-node/ &secure_storage_partition;
+
+&cpuppr_vpr {
+	status = "disabled";
+	/delete-property/ source-memory;
+	/delete-property/ execution-memory;
+};
+
+&cpuflpr_vpr {
+	status = "disabled";
+	/delete-property/ source-memory;
+	/delete-property/ execution-memory;
+};
+
+&mram1x {
+	partitions {
+		cpuapp_boot_partition: partition@312000 {
+			compatible = "zephyr,mapped-partition";
+			reg = <0x312000 DT_SIZE_K(64)>;
+		};
+
+		cpuapp_slot0_partition: partition@322000 {
+			compatible = "zephyr,mapped-partition";
+			reg = <0x322000 DT_SIZE_K(888)>;
+		};
+
+		storage_partition: partition@400000 {
+			compatible = "zephyr,mapped-partition";
+			reg = <0x400000 DT_SIZE_K(40)>;
+		};
+
+		periphconf_partition: partition@40a000 {
+			compatible = "zephyr,mapped-partition";
+			reg = <0x40a000 DT_SIZE_K(8)>;
+		};
+
+		secure_storage_partition: partition@40c000 {
+			compatible = "zephyr,mapped-partition";
+			reg = <0x40c000 DT_SIZE_K(8)>;
+			ranges = <0x0 0x40c000 0x2000>;
+			#address-cells = <1>;
+			#size-cells = <1>;
+
+			cpuapp_crypto_partition: partition@0 {
+				compatible = "zephyr,mapped-partition";
+				reg = <0x0 DT_SIZE_K(4)>;
+			};
+
+			cpuapp_its_partition: partition@1000 {
+				compatible = "zephyr,mapped-partition";
+				reg = <0x1000 DT_SIZE_K(4)>;
+			};
+		};
+	};
+};