devicetree: watchdog: add binding for Sigma Designs SMP8642 watchdog

This adds a binding for the watchdog in Sigma Designs SMP8642 and
similar devices.

Signed-off-by: Mans Rullgard <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

Documentation/devicetree/bindings/watchdog/sigma,smp8642-wdt.txt

···
+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>;
+};