drivers: net: phy: mdio-xgene: Add hardware dependency

The mdio-xgene driver is only useful on X-Gene SoC.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Iyappan Subramanian <isubramanian@apm.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>