dt: bindings: add documentation for zx2967 family watchdog controller

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>;

};