net: bcmgenet: Add dependency on HAS_IOMEM && OF

The driver needs CONFIG_HAS_IOMEM and OF to be functional, but we still
let it build with COMPILE_TEST. This fixes the unmet dependency after
selecting MDIO_BCM_UNIMAC in commit mentioned below:

warning: (NET_DSA_BCM_SF2 && BCMGENET) selects MDIO_BCM_UNIMAC which has
unmet direct dependencies (NETDEVICES && MDIO_DEVICE && HAS_IOMEM &&
OF_MDIO)

Fixes: 9a4e7969

 ("net: bcmgenet: utilize generic Broadcom UniMAC MDIO controller driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>