net: sh_eth: add set_mdio_gate in bb_info

The SH7757's ETHER and GETHER use common MDIO pin. The MDIO pin is
selected by specific register. So this patch adds new interface in
bb_info, and when the sh_eth driver use the mdio, the register can
be changed by the function.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>