net: atlantic: A2: half duplex support

This patch adds support for 10M/100M/1G half duplex rates, which are
supported by A2 in additional to full duplex rates supported by A1.

Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>