In case of i.MX6, there is detail description about unused pin treatment in Hardware development guide doc but about i.MX53, it seems to be not specified in datasheet or design guide.
Basically, I guess unused GPIO should be left unconnected and unused DDR IO also can be left unconnected. How about other analog signals such as LVDS, TVDAC, or SATA? There is some other signals for CKIH and PMIC_ON/STBY_REQ which is not used in customer design.
Could you let me know how it can be treated and correct me if there is any incorrect understanding.