devicetree: watchdog: add binding for Sigma Designs SMP8642 watchdog

Sigma Designs SMP86xx/SMP87xx watchdog

Required properties:

- compatible: Should be "sigma,smp8642-wdt"

- reg: Specifies the physical address region

- clocks: Should be a phandle to the clock

Optional properties:

- timeout-sec: watchdog timeout in seconds

Example:

watchdog@1fd00 {

	compatible = "sigma,smp8642-wdt";

	reg = <0x1fd00 8>;

	clocks = <&xtal_in_clk>;

	timeout-sec = <30>;

};