net: sh_eth: Add support R8A7740 of rmobile (arm core)

R8A7740 of rmobile has ethernet device, and this is same IP of
sh-ether. This support R8A7740 of rmobile.

Signed-off-by: Hideyuki Sano <hideyuki.sano.dn@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>