boards: stm32: watchdog enable watchdog support with dt

Now that stm32 watchdog should be configured by device tree,
update dts file of boards declaring watchdog support.
Additionally update doc and yaml files.
Add support on some boards that were used to validate the driver
update:
- disco_l475_iot1
- nucleo_f207zg
- nucleo_f429zi
- nucleo_f746zg
- nucleo_f073rz


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