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

Add i.MX system controller watchdog binding doc.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

Documentation/devicetree/bindings/watchdog/fsl-imx-sc-wdt.txt

+* 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>;
+};