drivers/net/phy/Kconfig: Let MDIO_BCM_UNIMAC depend on HAS_IOMEM

MDIO_BCM_UNIMAC needs HAS_IOMEM, so depend on it, the related error (
with allmodconfig under um):

    MODPOST 1205 modules
  ERROR: "devm_ioremap" [drivers/net/phy/mdio-bcm-unimac.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>