dt-bindings: add mt6592 compatible string for mediatek sysirq

This patch adds a compatible string for mt6592 SoC to the dts documentation of
mediateks sysirq.
Signed-off-by: Howard Chen <howard.chen@linaro.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

Documentation/devicetree/bindings/arm/mediatek/mediatek,sysirq.txt

@@ -7,6 +7,7 @@ Required properties:
 - compatible: should be one of:
 	"mediatek,mt8135-sysirq"
 	"mediatek,mt8127-sysirq"
+	"mediatek,mt6592-sysirq"
 	"mediatek,mt6589-sysirq"
 	"mediatek,mt6582-sysirq"
 	"mediatek,mt6577-sysirq"