dt-bindings: watchdog: Add Spreadtrum watchdog documentation

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>;

	};