dt-bindings: watchdog: Add Spreadtrum watchdog documentation

This patch adds the documentation for Spreadtrum watchdog driver.
Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

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

+Spreadtrum SoCs Watchdog timer
+
+Required properties:
+- compatible : Should be "sprd,sp9860-wdt".
+- reg : Specifies base physical address and size of the registers.
+- interrupts : Exactly one interrupt specifier.
+- timeout-sec : Contain the default watchdog timeout in seconds.
+- clock-names : Contain the input clock names.
+- clocks : Phandles to input clocks.
+
+Example:
+	watchdog: watchdog@40310000 {
+		compatible = "sprd,sp9860-wdt";
+		reg = <0 0x40310000 0 0x1000>;
+		interrupts = <GIC_SPI 61 IRQ_TYPE_LEVEL_HIGH>;
+		timeout-sec = <12>;
+		clock-names = "enable", "rtc_enable";
+		clocks = <&clk_aon_apb_gates1 8>, <&clk_aon_apb_rtc_gates 9>;
+	};