sandbox: watchdog: Avoid an error on startup

For some time now running sandbox with -T produces an error:

   Core:  270 devices, 95 uclasses, devicetree: board
   WDT:   Not starting wdt-gpio-toggle
   wdt_gpio wdt-gpio-level: Request for wdt gpio failed: -16
   WDT:   Not starting wdt@0
   MMC:   mmc2: 2 (SD), mmc1: 1 (SD), mmc0: 0 (SD)

Use an unallocated GPIO to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 1fc45d6483d ("watchdog: add pulse support to gpio watchdog driver")
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index e88c267..e430347 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -975,7 +975,7 @@
 	};
 
 	wdt-gpio-toggle {
-		gpios = <&gpio_a 7 0>;
+		gpios = <&gpio_a 8 0>;
 		compatible = "linux,wdt-gpio";
 		hw_margin_ms = <100>;
 		hw_algo = "toggle";
diff --git a/test/dm/wdt.c b/test/dm/wdt.c
index 653d7b1..2bbebcd 100644
--- a/test/dm/wdt.c
+++ b/test/dm/wdt.c
@@ -54,7 +54,7 @@
 	 */
 	struct udevice *wdt, *gpio;
 	const u64 timeout = 42;
-	const int offset = 7;
+	const int offset = 8;
 	int val;
 
 	ut_assertok(uclass_get_device_by_name(UCLASS_WDT,
@@ -115,7 +115,7 @@
 	struct udevice *gpio_wdt, *sandbox_wdt;
 	struct udevice *gpio;
 	const u64 timeout = 42;
-	const int offset = 7;
+	const int offset = 8;
 	uint reset_count;
 	int val;