[nrf fromtree] samples: drivers: watchdog: Enable sample on nrf54L09 and L20

Add overlays required to run watchdog sample on:
- nrf54l09pdk/nrf54l09/cpuapp,
- nrf54l09pdk/nrf54l09/cpuflpr,
- nrf54l20pdk/nrf54l20/cpuflpr.

Signed-off-by: Sebastian Głąb <[email protected]>
(cherry picked from commit a9e822e)

Author: nordic-segl

samples/drivers/watchdog/boards/nrf54l09pdk_nrf54l09_cpuapp.overlay

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&wdt31 {
+	status = "okay";
+};

samples/drivers/watchdog/boards/nrf54l09pdk_nrf54l09_cpuflpr.overlay

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&wdt31 {
+	status = "okay";
+};

samples/drivers/watchdog/boards/nrf54l20pdk_nrf54l20_cpuflpr.overlay

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&wdt31 {
+	status = "okay";
+};