samples/driver: watchdog: wwdg sample should run with west

A different configuration is applied when running this test
using west or using twister.
Both will use the disco_l475_iot1 overlay, but the extra config
for clock bus tunning is applied only when using twister and
samples is failed when run with west.
Move this extra config from sample.yaml to board .conf file so
it is applied in both cases.

Fixes #32376

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>

samples/drivers/watchdog/boards/disco_l475_iot1.conf

@@ -0,0 +1 @@
+CONFIG_CLOCK_STM32_APB1_PRESCALER=16

samples/drivers/watchdog/sample.yaml

@@ -28,5 +28,3 @@ tests:
     tags: drivers watchdog
     filter: dt_compat_enabled("st,stm32-window-watchdog")
     depends_on: watchdog
-    extra_configs:
-      - CONFIG_CLOCK_STM32_APB1_PRESCALER=16