ARM: dts: keystone: fix dt bindings to use post div register for mainpll

All of the keystone devices have a separate register to hold post
divider value for main pll clock. Currently the fixed-postdiv
value used for k2hk/l/e SoCs works by sheer luck as u-boot happens to
use a value of 2 for this. Now that we have fixed this in the pll
clock driver change the dt bindings for the same.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>