drivers: watchdog: wdt_nrfx: Implement disable API

nRF5340 SoC has `TASK_STOP` this patch implements disabling
watchdog for that SoC.

Changed body of `wdt_nrf_setup()` to utilize `nrfx_wdt_reconfigure()`
driver API.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>