board: x530: switch to wdt_stop_all()

Since the gd->watchdog_dev member is going away, switch to using the
new wdt_stop_all() helper.

While here, clean up the preprocessor conditional: The ->watchdog_dev
member is actually guarded by CONFIG_WDT [disabling that in
x530_defconfig while keeping CONFIG_WATCHDOG breaks the build], and in
the new world order so is the existence of the wdt_stop_all()
function.

Actually, existence of wdt_stop_all() depends on CONFIG_${SPL_}WDT, so
really spell the condition using CONFIG_IS_ENABLED, and make it a C
rather than cpp if.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>