net: ethernet: mtk-star-emac: store bit_clk_div in compat structure

Not all the SoC are using the same clock divider. Move the divider into
a compat structure specific to the SoCs.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>