Hi,
You need to create a so called board driver that describes how the ad1938 is connected to the I2S module of the RPI.
For a simple example of such a board driver see for example this file. To make it work with the ad1938 you need to update the codec_name and codec_dai_name fields of the snd_soc_dai_link struct. The codec_name field needs to be set to the name your spi device as it appears in /sys/bus/spi/devices (e.g. spi1.0) and the codec_dai_name field needs to be set to ad193x-hifi.
For more information please see the AD1938 Linux driver documentation.
- Lars