net: ethernet: litex: Add the dependency on HAS_IOMEM

The LiteX driver uses devm io function API which
needs HAS_IOMEM enabled, so add the dependency on HAS_IOMEM.

Fixes: ee7da21a

 ("net: Add driver for LiteX's LiteETH network interface")
Signed-off-by: Cai Huoqing <cai.huoqing@linux.dev>
Link: https://lore.kernel.org/r/20220208013308.6563-1-cai.huoqing@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>