boot: zephyr: setup watchdog

The Zephyr watchdog API defines a setup function. This function needs to
be executed before the watchdog is functional in some cases. This commit
adds MCUBOOT_WATCHDOG_SETUP when using the generic watchdog0 alias
otherwise it is an empty define.

Fixes https://github.com/mcu-tools/mcuboot/issues/1659



Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>