watchdog: stm32_iwdg: fix error message during driver probe

The commit 3ab1663a ("watchdog: stm32_iwdg: Add pretimeout support")
introduces the support for the pre-timeout interrupt.

The support for this interrupt is optional but the driver uses the
platform_get_irq() which produces an error message during the driver
probe if we don't have any `interrupts` property in the DT.

Use the platform_get_irq_optional() API to get rid of the error message
as this property is optional.

Fixes: 3ab1663a ("watchdog: stm32_iwdg: Add pretimeout support")
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20241218092227.771133-1-clement.legoffic@foss.st.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>