clk: keystone: use clkod register bits for postdiv
DDR3A/B, ARM and PA PLL controllers have clkod register bits for
configuring postdiv values. So use it instead of using fixed
post dividers for these pll controllers. Assume that if fixed-postdiv
attribute is not present, use clkod register value for pistdiv.
Also update the Documentation of bindings to reflect the same.