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>