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>
diff --git a/drivers/watchdog/Kconfig.sam b/drivers/watchdog/Kconfig.sam
index 9cfd452..3556a46 100644
--- a/drivers/watchdog/Kconfig.sam
+++ b/drivers/watchdog/Kconfig.sam
@@ -7,5 +7,6 @@
 	bool "Atmel SAM MCU Family Watchdog (WDT) Driver"
 	default y
 	depends on DT_HAS_ATMEL_SAM_WATCHDOG_ENABLED
+	select HAS_WDT_DISABLE_AT_BOOT
 	help
 	  Enable WDT driver for Atmel SAM MCUs.