watchdog: bindings: Add binding documentation for Toshiba Visconti watchdog device

Add documentation for the binding of Toshiba Visconti SoC's watchdog.
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Reviewed-by: Punit Agrawal <punit1.agrawal@toshiba.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201005023012.603026-2-nobuhiro1.iwamatsu@toshiba.co.jpSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/devicetree/bindings/watchdog/toshiba,visconti-wdt.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2020 Toshiba Electronic Devices & Storage Corporation
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/watchdog/toshiba,visconti-wdt.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Toshiba Visconti SoCs PIUWDT Watchdog timer
+
+maintainers:
+  - Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
+
+allOf:
+  - $ref: watchdog.yaml#
+
+properties:
+  compatible:
+    enum:
+      - toshiba,visconti-wdt
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  timeout-sec: true
+
+required:
+  - compatible
+  - reg
+  - clocks
+
+additionalProperties: false
+
+examples:
+  - |
+    soc {
+        #address-cells = <2>;
+        #size-cells = <2>;
+
+        wdt_clk: wdt-clk {
+            compatible = "fixed-clock";
+            clock-frequency = <150000000>;
+            #clock-cells = <0>;
+        };
+
+        watchdog@28330000 {
+            compatible = "toshiba,visconti-wdt";
+            reg = <0 0x28330000 0 0x1000>;
+            clocks = <&wdt_clk>;
+            timeout-sec = <20>;
+        };
+    };