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.

Cc: Mike Turquette <mturquette@linaro.org
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>