watchdog: qcom: document device tree bindings

The Qualcomm Krait Processor Sub-system (KPSS) contains one or more
instances of the WDT.  Provide documentation on how to describe these in
the device tree.

Signed-off-by: Josh Cartwright <joshc@codeaurora.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

Documentation/devicetree/bindings/watchdog/qcom-wdt.txt

+Qualcomm Krait Processor Sub-system (KPSS) Watchdog
+---------------------------------------------------
+
+Required properties :
+- compatible : shall contain only one of the following:
+
+			"qcom,kpss-wdt-msm8960"
+			"qcom,kpss-wdt-apq8064"
+			"qcom,kpss-wdt-ipq8064"
+
+- reg : shall contain base register location and length
+- clocks : shall contain the input clock
+
+Optional properties :
+- timeout-sec : shall contain the default watchdog timeout in seconds,
+                if unset, the default timeout is 30 seconds
+
+Example:
+	watchdog@208a038 {
+		compatible = "qcom,kpss-wdt-ipq8064";
+		reg = <0x0208a038 0x40>;
+		clocks = <&sleep_clk>;
+		timeout-sec = <10>;
+	};