net: ethernet: Add missing MFD_SYSCON dependency on HAS_IOMEM

The MFD_SYSCON depends on HAS_IOMEM so when selecting it avoid unmet
direct dependencies.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>