what clock do you need support?
downclock 80%
try to use the command
echo 5 > /sys/devices/soc0/soc/soc\:busfreq/enable
if current PLL for your DDR couldn't support frequency you need, maybe you need change the PLL in uboot and kernel
You can change the dts default settings fsl,max_ddr_freq = <400000000>; to see if it can work or not.
But need to the pll freq can be divided.