drivers: watchdog: Align nrf wdt driver to new nrfx
drivers: watchdog: Align nrf wdt driver to new nrfx

New nrfx version brings fixes also in watchdog implementation.
Previously introduced workaround for supporting more than one channel
can be changed to driver-management solution.

Signed-off-by: Karol Lasończyk <72ec7e01745ba68db3ccaea40b82002ff17da530@nordicsemi.no>

punitvara/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,kraj/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,kraj/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,galak/zephyr,explora26/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr