net/sh-eth: Add support selecting MII function for SH7734 and R8A7740

Ethernet IP of SH7734 and R8A7740 has selecting MII register.
The user needs to change a value according to MII to be used.
This adds the function to change the value of this register.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>