net: sh-eth: Add support R8A7793

R8A7793 has the same sh-ether IP core as other SH/rmobile.
This patch adds support of R8A7793.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>