net: sh-eth: Add support R8A7790

R8A7790 has the same sh-ether IP core as other SH/rmobile.
This patch adds support of R8A7790.

Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>