Hi Tobias,
If you want to call a DMA transfer when I2S received data, the I2S Rx is the DMA source. Conversely, if you want to call a DMA transfer when I2S need more data, the I2S Tx is the DMA source.There is a DMA request source table in K20P100M72SF1RM.pdf page 80. You can set corresponding number to DMAMUX's register.
There are some resource you can refer to. If you have download K20 SDK package, please refer to SDK_2.2_MK20DN512xxx10\boards\twrk60d100m\driver_examples\sai\edma_transfer.
Although it works in SAI mode, the dma setting is useful.
You can also refer to AN4800 which you can find on NXP website.
Hope my suggesting will be useful to you
Regards