drivers: watchdog: Rename CONFIG_WDT_SAM_DISABLE_AT_BOOT in comments
drivers: watchdog: Rename CONFIG_WDT_SAM_DISABLE_AT_BOOT in comments

CONFIG_WDT_SAM_DISABLE_AT_BOOT was removed in commit 2e01e86bdc
("drivers: watchdog: wdt_sam: use the generic disable option"), but some
comments still talked about it. Replace it with
CONFIG_WDT_DISABLE_AT_BOOT.

Signed-off-by: Ulf Magnusson <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr