net: calxedaxgmac: fix driver dependencies

Calxeda 1G/10G XGMAC Ethernet support should be available only on
Calxeda ECX-1000/2000 (Highbank/Midway) platforms.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>