net: sh_eth: add support for SH7757's GETHER

SH7757 has 2 ETHERs and 2 GETHERs. This patch supports the SH7757's
GETHER. If CONFIG_SH_ETHER_USE_GETHER is defined using SH7757,
the driver handles the GETHER.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>