net: sh_eth: modify the definitions of regsiter

The previous code had many similar definitions in each CPU.

This patch borrows from the sh_eth driver of Linux kernel.

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