net: calxeda xgmac ethernet driver add missing HAS_IOMEM dependency

Fix allyesconfig build on architectures without IOMEM:

drivers/net/ethernet/calxeda/xgmac.c:1800:2:
  error: implicit declaration of function 'iounmap' [-Werror=implicit-function-declaration]

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Rob Herring <rob.herring@calxeda.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>