net: ethernet: arc: Keep emac compatibility for more Rockchip SoCs

On the RK3066/RK3188, there was fixed GRF offset configuration to set emac
and fixed DIV2 mac TX/RX clock. So, we need to easily set and fit to other
SoCs (RK3036) which maybe have different GRF offset, and need adjust mac
TX/RX clock.

Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>