net: stmmac: constify clk_div_table

clk_div_table are not supposed to change at runtime.
meson8b_dwmac structure is working with const clk_div_table.
So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>