net: xgene Remove xgene specific phy and MAC lookup functions

Convert the xgene_get_mac_address to device_get_mac_address(), and
xgene_get_phy_mode() to device_get_phy_mode().

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>