Revert "drivers: watchdog: Make WDT_DISABLE_AT_BOOT default to y"

This reverts commit 382e6fbc.

Disabling watchdog at boot breaks watchdog API contract. Production
firmware should never ship with the option enabled. Unfortunately, this
is dangerosly easy to overlook. If left enabled, in the best case, it
will degrade functionality of the subsystem. In the worst case it
will leave watchdog permanently disbled.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>