zbt-wd323: add GPIO WDT support

zbt-wd323: add GPIO WDT support

Watchdog has not been properly configured for this router - the PCB has a
hardware watchdog connected to one of the GPIO pin 21 [1]
This commit provides this fix [2]

Without this fix, the ZBT-WD323 is unusable in OpenWRT because it power
cycles every 30 seconds due to the watchdog tripping

[1] https://forum.openwrt.org/t/zbt-wd323-router-power-cycles-every-30-seconds/77535/7
[2] https://forum.openwrt.org/t/zbt-wd323-images-unusable-proposed-workaround/162145/5

Signed-off-by: Joshua O'Leary <josh.oleary@mobile-power.co.uk>