mips: dts: ralink: mt7621: rename watchdog node from 'wdt' into 'watchdog'

Watchdog nodes must use 'watchdog' for node name. When a 'make dtbs_check'
is performed the following warning appears:

wdt@100: $nodename:0: 'wdt@100' does not match '^watchdog(@.*|-[0-9a-f])?$'

Fix this warning up properly renaming the node into 'watchdog'.

Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>