dt-bindings: watchdog: imx7ulp-wdt: Add imx8ulp compatible string

The wdog on i.MX8ULP is derived from i.MX7ULP, it uses two compatible
strings, so update the compatible string for i.MX8ULP.
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211112082930.3809351-7-peng.fan@oss.nxp.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/devicetree/bindings/watchdog/fsl-imx7ulp-wdt.yaml

@@ -14,8 +14,11 @@ allOf:
 
 properties:
   compatible:
-    enum:
-      - fsl,imx7ulp-wdt
+    oneOf:
+      - const: fsl,imx7ulp-wdt
+      - items:
+          - const: fsl,imx8ulp-wdt
+          - const: fsl,imx7ulp-wdt
 
   reg:
     maxItems: 1