net: dsa: lantiq_gswip: Depend on HAS_IOMEM

The driver uses devm_ioremap_resource() which is only available when
CONFIG_HAS_IOMEM is set, make the driver depend on this config option.
User mode Linux does not have CONFIG_HAS_IOMEM set and the driver was
failing on this architecture.

Fixes: 14fceff4 ("net: dsa: Add Lantiq / Intel DSA driver for vrx200")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>