ath79: dts: disable redundant built-in watchdog

ath79: dts: disable redundant built-in watchdog

The built-in watchdog is redundant when the device has an external
GPIO based hardware watchdog. And there is a conflict that both of
them will attempt to register the same device entry in sysfs. This
resulted in the built-in watchdog being unable to be activated.
This patch explicitly disables the built-in watchdog for devices
that use GPIO watchdog to fix the error:

[    1.779206] ath79-wdt 18060008.wdt: unable to register misc device, err=-16
[    1.786355] ath79-wdt: probe of 18060008.wdt failed with error -16

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18395
Signed-off-by: Robert Marko <robimarko@gmail.com>