net: axienet: mark as a legacy_pre_march2020 driver

axienet has a PCS, but does not make use of the phylink PCS support.
Mark it was a pre-March 2020 driver.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>