dts: nrf: Rename "nordic,nrf-watchdog" compatible to "nordic,nrf-wdt"

For consistency with the scheme used for other nRF peripherals,
use the peripheral name that nRF Product Specifications use.
In this case, it is WDT, not WATCHDOG.

Also remove the requirement for the label property in the binding.
It is no longer needed.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>