I think it is really weird that it works.
And even without trying anything, I guess the work flow would be :
backup everything :p
Open the pinmux file (with Windows OS), change user usage column (AE) from unused to the function in DDR3 lanes (from row 233).
Do this for all signal you want (depends on your wish to keep either chips 1&2 or chips 3&4 in the clamshell configuration).
generate the dtsi files by clicking on the button (column AE row 1 and macros should be activated)
copy/collate those files in your uboot\dts
change emc.c under u-boot\arch\arm\mach-tegra\tegra20\ :
modify the EMC_FBIO_CFG5_0 (bit 4 : change it from 1 to 0 for x32 PMC support and write the value of bit 5 to your desired value). I’m not sure it is the one because Tegra20 is not the one on Jetson…but who knows…
5)build your new image and flash it
Edit : maybe some other configuration to work with (.par file for your memory coming with the Memory Characterization Tool) and other kernel conf I don’t really know how to deal with… :/