net: axienet: Add optional support for Ethernet core interrupt

Previously this driver only handled interrupts from the DMA RX and TX
blocks, not from the Ethernet core itself. Add optional support for
the Ethernet core interrupt, which is used to detect rx_missed and
framing errors signalled by the hardware. In order to use this
interrupt, a third interrupt needs to be specified in the device tree.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>