zephyr: ci: enable testing more watchdog paths

Enable tests to be run on frdm_k64f and disco_l475_iot1. The l475 uses
the STM32 IWDG by default, and the k64f can be used for the generic
watchdog path. Both boards also received a config to enable the
watchdog.

Signed-off-by: Fabio Utzig <utzig@apache.org>