net/cadence: depend on HAS_IOMEM

Fixes the following build failure on S390:
  In file included from drivers/net/ethernet/cadence/at91_ether.c:35:0:
   drivers/net/ethernet/cadence/macb.h: In function 'macb_is_gem':
   drivers/net/ethernet/cadence/macb.h:563:2: error: implicit declaration of function '__raw_readl' [-Werror=implicit-function-declaration]
   drivers/net/ethernet/cadence/at91_ether.c: In function 'update_mac_address':
   drivers/net/ethernet/cadence/at91_ether.c:119:2: error: implicit declaration of function '__raw_writel' [-Werror=implicit-function-declaration]
   cc1: some warnings being treated as errors

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>