There is no I2S support in the R2 release, however if you take a look at the new Edison hardware guide you can see information of i2S https://communities.intel.com/docs/DOC-23158 . There you will find information related to the i2S interface pins and the different modes.
Now, in that document you can also find useful information of SPI. My recommendation would be to use SPI, since you can find more of information on SPI in documents and in this forum.
The tables with the SPI pins of the Edison are available in table 5.
Now why don’t you want to use USB? Using USB is probably the most straightforward way to connect the DAC.