drivers: watchdog: Watchdog API redesign

New API enables setting watchdog timeout in the unit of microseconds.
It is possible to configure watchdog timer behavior in CPU sleep state
as well as when it is halted by the debugger.
It supports watchdogs with multiple reload channels.

Jira: ZEP-2564

Signed-off-by: Michał Kruszewski <michal.kruszewski@nordicsemi.no>
Signed-off-by: Karol Lasończyk <karol.lasonczyk@nordicsemi.no>