[nrf fromlist] samples: drivers: spi_flash: Enable sample on nrf54lm20dk

Add overlay required to run the sample on
nrf54lm20dk/nrf54lm20a/cpuapp.

Upstream PR #: 92432

Signed-off-by: Sebastian Głąb <[email protected]>

Author: nordic-segl

samples/drivers/spi_flash/boards/nrf54lm20dk_nrf54lm20a_cpuapp.overlay

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&mx25r64 {
+	status = "okay";
+};

samples/drivers/spi_flash/sample.yaml

@@ -22,3 +22,18 @@ tests:
         - "Attempting to write 4 bytes"
         - "Data read matches data written. Good!!"
     depends_on: spi
+  sample.drivers.spi.flash.nrf54lm20:
+    platform_allow:
+      - nrf54lm20dk/nrf54lm20a/cpuapp
+    integration_platforms:
+      - nrf54lm20dk/nrf54lm20a/cpuapp
+    harness_config:
+      fixture: external_flash
+      type: multi_line
+      ordered: true
+      regex:
+        - "Test 1: Flash erase"
+        - "Flash erase succeeded!"
+        - "Test 2: Flash write"
+        - "Attempting to write 4 bytes"
+        - "Data read matches data written. Good!!"