watchdog: wdt-uclass.c: add wdt_set_force_autostart() helper

watchdog: wdt-uclass.c: add wdt_set_force_autostart() helper

The watchdog could have been already started by a previous boot
stage (e.g. bootrom or secure OS). U-Boot has to start and kick
the watchdog even when CONFIG_WATCHDOG_AUTOSTART is not enabled
or when the DT property u-boot,noautostart is present.

Add the helper wdt_set_force_autostart() that can be called by the
driver's probe() when it detects that the watchdog has already
been started and is running.

Co-developed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>