Document: DT: Add bindings for mediatek MT8173 SoC Platform

MT8173 is a 64-bit four core Soc.
And mt8173-evb is a evaluation board based on mt8173.
This commit add the devicetree binding document for
  mediatek MT8173 SoC
  MT8173 sysirq
  MT8173 uart

Signed-off-by: Eddie Huang <eddie.huang@mediatek.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>

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

@@ -5,6 +5,7 @@ interrupt.
 
 Required properties:
 - compatible: should be one of:
+	"mediatek,mt8173-sysirq"
 	"mediatek,mt8135-sysirq"
 	"mediatek,mt8127-sysirq"
 	"mediatek,mt6589-sysirq"