MIPS: ralink: mt7628a.dtsi: Add watchdog controller DT node

This patch adds the watchdog controller description to the MT7628A dtsi
file.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Harvey Hunt <harveyhuntnexus@gmail.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org