dt: bindings: add documentation for zx2967 family watchdog controller

This patch adds dt-binding documentation for zx2967 family
watchdog controller.
Signed-off-by: NBaoyou Xie <baoyou.xie@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/watchdog/zte,zx2967-wdt.txt

+ZTE zx2967 Watchdog timer
+
+Required properties:
+
+- compatible : should be one of the following.
+       * zte,zx296718-wdt
+- reg : Specifies base physical address and size of the registers.
+- clocks : Pairs of phandle and specifier referencing the controller's clocks.
+- resets : Reference to the reset controller controlling the watchdog
+           controller.
+
+Optional properties:
+
+- timeout-sec : Contains the watchdog timeout in seconds.
+- zte,wdt-reset-sysctrl : Directs how to reset system by the watchdog.
+	if we don't want to restart system when watchdog been triggered,
+	it's not required, vice versa.
+	It should include following fields.
+	  * phandle of aon-sysctrl.
+	  * offset of register that be written, should be 0xb0.
+	  * configure value that be written to aon-sysctrl.
+	  * bit mask, corresponding bits will be affected.
+
+Example:
+
+wdt: watchdog@1465000 {
+	compatible = "zte,zx296718-wdt";
+	reg = <0x1465000 0x1000>;
+	clocks = <&topcrm WDT_WCLK>;
+	resets = <&toprst 35>;
+	zte,wdt-reset-sysctrl = <&aon_sysctrl 0xb0 1 0x115>;
+};