dt-bindings: watchdog: add i.MX system controller watchdog

* Freescale i.MX System Controller Watchdog

i.MX system controller watchdog is for i.MX SoCs with system controller inside,

the watchdog is managed by system controller, users can ONLY communicate with

system controller from secure mode for watchdog operations, so Linux i.MX system

controller watchdog driver will call ARM SMC API and trap into ARM-Trusted-Firmware

for watchdog operations, ARM-Trusted-Firmware is running at secure EL3 mode and

it will request system controller to execute the watchdog operation passed from

Linux kernel.

Required properties:

- compatible:	Should be :

		"fsl,imx8qxp-sc-wdt"

		followed by "fsl,imx-sc-wdt";

Optional properties:

- timeout-sec : Contains the watchdog timeout in seconds.

Examples:

watchdog {

	compatible = "fsl,imx8qxp-sc-wdt", "fsl,imx-sc-wdt";

	timeout-sec = <60>;

};