Do not recommend you to use the 16bit ddr3l, recommend you to use the 32bit ddr3l so that the i.MX6SX can reach to the best performance. Here if you still want to use the 16bit ddr3l, use the DRAM_DATA00-DRAM_DATA15 pins, let the DRAM_DATA16-DRAM_DATA31 pins floating.
Do not recommend you to use the 16bit ddr3l, recommend you to use the 32bit ddr3l so that the i.MX6SX can reach to the best performance. Here if you still want to use the 16bit ddr3l, use the DRAM_DATA00-DRAM_DATA15 pins, let the DRAM_DATA16-DRAM_DATA31 pins floating.