sandbox.dtsi: add a sandbox,alarm-wdt instance

In order to test that we properly handle watchdog(s) during the "wait
for the user to interrupt autoboot" phase, we need a watchdog device
to be watching us.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
diff --git a/arch/sandbox/dts/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index d2db1ea..fc5bde4 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -27,6 +27,12 @@
 		};
 	};
 
+	alarm_wdt: alarm-wdt {
+		compatible = "sandbox,alarm-wdt";
+		timeout-sec = <5>;
+		u-boot,autostart;
+	};
+
 	audio: audio-codec {
 		compatible = "sandbox,audio-codec";
 		#sound-dai-cells = <1>;