net,marvell: Add dependency on HAS_IOMEM

On archs like S390 or um this driver cannot build nor work.
Make it depend on HAS_IOMEM to bypass build failures.

drivers/built-in.o: In function `orion_mdio_probe':
drivers/net/ethernet/marvell/mvmdio.c:228: undefined reference to `devm_ioremap'

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: David S. Miller <davem@davemloft.net>