drivers: watchdog: Allow WDT_DISABLE_AT_BOOT only when supported

Introduce a hidden Kconfig option named HAS_WDT_DISABLE_AT_BOOT and
allow users to enable WDT_DISABLE_AT_BOOT only when that hidden option
is selected by a watchdog driver, i.e. disabling at boot is supported.
Select this new hidden option for all existing watchdog drivers that
refer to WDT_DISABLE_AT_BOOT.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>