net: ethernet: stmmac: don't warn about missing optional wakeup IRQ

The "stm32_pwr_wakeup" is optional per the binding and the driver
handles its absence gracefully. Request it with
platform_get_irq_byname_optional, so its absence doesn't needlessly
clutter the log.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>