net: xgene_enet: Set hardware dependency

The xgene_enet driver is only useful on X-Gene SoC.

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