[nrf fromlist] samples: watchdog: Add nrf9280 iron to sample

Added overlay for nrf9280 cpuapp iron board.

Upstream PR #: 90756
Signed-off-by: Aymen LAOUINI <[email protected]>

Author: ayla-nordicsemi

samples/drivers/watchdog/boards/nrf9280pdk_nrf9280_cpuapp_iron.overlay

@@ -0,0 +1,8 @@
+/*
+ * Copyright 2025 Nordic Semiconductor ASA
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&wdt010 {
+	status = "okay";
+};