samples/dfu/firmware_loader: partitioning of nrf54lm20dongle

Added DTS overlay for partitions on nRF54LM20dongle.
This is for firmware loader - slot0_parttion spans till end of RRAM.
MCUboot Is (and must use same partitioning).
An application build must keep the same start address but
can downsize the partition.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

Author: nvlsianpu

samples/dfu/fw_loader/ble_mcumgr/boards/nrf54lm20dongle_nrf54lm20b_cpuapp.overlay

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+/* This is nRF54lm20dongle_nrf54lm20b_cpuapp-specific partitioning for MCUboot and firmware loader.
+ * slot0_partition defines RRAM region which can be updated by the firmware loader and booted by
+ * MCUboot.
+ * Beginning of the slot0_partition must match the beginning of the slot0_partition of the user
+ * application, but the application can use a smaller partition and leave some space for
+ * other purposes like a storage partition.
+ */
+
+/delete-node/ &storage_partition;
+
+&cpuapp_rram {
+	partitions {
+		slot0_partition: partition@23000 {
+			label = "image-0";
+			reg = <0x23000 DT_SIZE_K(1896)>;
+		};
+	};
+};

samples/dfu/fw_loader/usb_mcumgr/boards/nrf54lm20dongle_nrf54lm20b_cpuapp.overlay

@@ -19,3 +19,22 @@
 };
 
 #include "../app.overlay"
+
+/* This is nRF54lm20dongle_nrf54lm20b_cpuapp-specific partitioning for MCUboot and firmware loader.
+ * slot0_partition defines RRAM region which can be updated by the firmware loader and booted by
+ * MCUboot.
+ * Beginning of the slot0_partition must match the beginning of the slot0_partition of the user
+ * application, but the application can use a smaller partition and leave some space for
+ * other purposes like a storage partition.
+ */
+
+/delete-node/ &storage_partition;
+
+&cpuapp_rram {
+	partitions {
+		slot0_partition: partition@23000 {
+			label = "image-0";
+			reg = <0x23000 DT_SIZE_K(1896)>;
+		};
+	};
+};