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

drivers/net/ethernet/calxeda/Kconfig

 config NET_CALXEDA_XGMAC
 	tristate "Calxeda 1G/10G XGMAC Ethernet driver"
-
+	depends on HAS_IOMEM
 	select CRC32
 	help
 	  This is the driver for the XGMAC Ethernet IP block found on Calxeda