soc: litex: Properly depend on HAS_IOMEM

The LiteX SOC controller driver makes use of IOMEM functions like
devm_platform_ioremap_resource(), which are only available if
CONFIG_HAS_IOMEM is defined.

This causes the driver to be enable under make ARCH=um allyesconfig,
even though it won't build.

By adding a dependency on HAS_IOMEM, the driver will not be enabled on
architectures which don't support it.

Fixes: 22447a99

 ("drivers/soc/litex: add LiteX SoC Controller driver")
Signed-off-by: David Gow <davidgow@google.com>
[shorne@gmail.com: Fix typo in commit message pointed out in review]
Signed-off-by: Stafford Horne <shorne@gmail.com>