I'm not so familiar with iMX53 as I'm working on imx51, but Here are some points:
I2C is used for configuration and setting up the codec (SGTL5000 is acting as a SLAVE Device so iMX provide a CLK).
I2S (or whatever they call it - different vendors, different implementation) is used for raw digital I2S audio path which then is converted to Analog on codec. Good datasheet is SGTL5000RM.pdf which is complete and full of information for different implementation. Both lines are connected to dedicated ports but you can use I2C port whichever you like as there's no problem to switch between them on driver side. I'm not familiar with cellular modules so I can't help you on that.
P.