dt-bindings: watchdog: Document armada-37xx-wdt binding

This adds device tree binding documentation for the CPU watchdog found
on Armada 37xx SOCs (EspressoBin, Turris Mox).

Signed-off-by: Marek Behún <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: [e-mail]
Reviewed-by: Rob Herring <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

Documentation/devicetree/bindings/watchdog/armada-37xx-wdt.txt

···
+* Armada 37xx CPU Watchdog Timer Controller
+
+Required properties:
+- compatible : must be "marvell,armada-3700-wdt"
+- reg : base physical address of the controller and length of memory mapped
+	region.
+- clocks : the clock feeding the watchdog timer. See clock-bindings.txt
+- marvell,system-controller : reference to syscon node for the CPU Miscellaneous
+	Registers
+
+Example:
+
+	cpu_misc: system-controller@d000 {
+		compatible = "marvell,armada-3700-cpu-misc", "syscon";
+		reg = <0xd000 0x1000>;
+	};
+
+	wdt: watchdog@8300 {
+		compatible = "marvell,armada-3700-wdt";
+		reg = <0x8300 0x40>;
+		marvell,system-controller = <&cpu_misc>;
+		clocks = <&xtalclk>;
+	};