ramips: fix missing mediatek wdt

mediatek MT7621 soc watchdog DTS id was renamed from "mtk,mt7621-wdt" to
"mediatek,mt7621-wdt" when driver upstreamed to kernel 4.5

Update mt7621.dtsi & mt7628an.dtsi definitions to match upstreamed
kernel.

Restores hardward watchdog functionality on mt7621 devices under linux
4.9

Tested on: MIR3G

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

target/linux/ramips/dts/mt7621.dtsi

@@ -58,7 +58,7 @@
 		};
 
 		wdt: wdt@100 {
-			compatible = "mtk,mt7621-wdt";
+			compatible = "mediatek,mt7621-wdt";
 			reg = <0x100 0x100>;
 		};
 

target/linux/ramips/dts/mt7628an.dtsi

@@ -38,7 +38,7 @@
 		};
 
 		watchdog: watchdog@120 {
-			compatible = "ralink,mt7628an-wdt", "mtk,mt7621-wdt";
+			compatible = "ralink,mt7628an-wdt", "mediatek,mt7621-wdt";
 			reg = <0x120 0x10>;
 
 			resets = <&rstctrl 8>;