net: axienet: Mark eth_irq as optional

According to the DT binding, the Ethernet core interrupt is optional.

Use platform_get_irq_optional() to avoid the error message when the
IRQ is not specified.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>