Hi Xiangjun,
Here are the reasons why i think K66 only supports 16 bit wide SDRAM blocks and its likely documentation error on NXP's account.
1. If you look at K66P144M180SF5V2.pdf, the hardware manual, look at Figure 15, it only shows data bits from 31-16.
2. If you go to section 5 of the same doc, the pinout section and multiplexing assignments, you can only find 16 bits for the SDRAM data lines.
3. Also see this thread.
https://community.nxp.com/thread/388002
I just wanted confirmation from someone from NXP. Looks like documentation still hasn't been updated.