dt-bindings: watchdog: mt7621-wdt: add phandle to access system controller registers

MT7621 SoC provides a system controller node for accessing to some registers.
Add a phandle in this node to avoid using MIPS related arch operations and
includes in watchdog driver code.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230214103936.1061078-2-sergio.paracuellos@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/devicetree/bindings/watchdog/mediatek,mt7621-wdt.yaml

@@ -19,6 +19,12 @@ properties:
   reg:
     maxItems: 1
 
+  mediatek,sysctl:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      phandle to system controller 'sysc' syscon node which
+      controls system registers
+
 required:
   - compatible
   - reg
@@ -30,4 +36,5 @@ examples:
     watchdog@100 {
       compatible = "mediatek,mt7621-wdt";
       reg = <0x100 0x100>;
+      mediatek,sysctl = <&sysc>;
     };