net: ethernet: broadcom: Drop dependency on OF

Both BCMGENET and SYSTEMPORT build just fine with CONFIG_OF=n, we do have a
dependency on HAS_IOMEM that was not being reflected for SYSTEMPORT so add
that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>