doc: mtk-wdt: support pre-timeout when the bark irq is available

Add description of pre-timeout in mtk-wdt.
Signed-off-by: Wang Qing <wangqing@vivo.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1619148020-2236-3-git-send-email-wangqing@vivo.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

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

 Mediatek SoCs Watchdog timer
 
+The watchdog supports a pre-timeout interrupt that fires timeout-sec/2
+before the expiry.
+
 Required properties:
 
 - compatible should contain:
@@ -17,6 +20,7 @@ Required properties:
 - reg : Specifies base physical address and size of the registers.
 
 Optional properties:
+- interrupts: Watchdog pre-timeout (bark) interrupt.
 - timeout-sec: contains the watchdog timeout in seconds.
 - #reset-cells: Should be 1.
 
@@ -26,6 +30,7 @@ watchdog: watchdog@10007000 {
 	compatible = "mediatek,mt8183-wdt",
 		     "mediatek,mt6589-wdt";
 	reg = <0 0x10007000 0 0x100>;
+	interrupts = <GIC_SPI 139 IRQ_TYPE_NONE>;
 	timeout-sec = <10>;
 	#reset-cells = <1>;
 };