Make the watchdog part of the driver optional, under a boolean config
option. Move the dependency to WATCHDOG to this new option, and change
the WATCHDOG_CORE dependency to selection, as is done in most watchdog
drivers.

This makes the turris-omnia-mcu driver available for compilation even if
WATCHDOG is disabled.

Fixes: ed46f1f7 ("platform: cznic: turris-omnia-mcu: fix Kconfig dependencies")
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20240719085756.30598-2-kabel@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>