powerpc/512x: Constify clk_div_tables

clk_div_tables are not supposed to change at runtime.
mpc512x_clk_divtable function 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: Michael Ellerman <mpe@ellerman.id.au>