net: sh_eth: Unify CONFIG_R8A779[01234] as CONFIG_RCAR_GEN2

Use the common RCAR_GEN2 config option instead of enumerating
each SoC and having a lengthy ifdef clause. No functional change.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Joe Hershberger <joe.hershberger@ni.com>