net: axienet: Add mii-tool support

mii-tool is useful for debugging, and all it requires to work is to wire
up the ioctl ops function pointer.
Add this to the axienet driver to enable mii-tool.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>