I2C has Master and Slave.
Master could be SoC I2C controller.
Slave could be EEPROM, LCD, Audio Codec.
So Linux should provide master driver and slave driver.
Master driver: how to send cmd/data to slave device
Slave driver: how to access slave register, data.
borrow a picture from https://blog.csdn.net/wangpengqi/article/details/17711165
In this picture, Client is slave, adaptor is master. Then everything is easy to understand.
To add Master/adaptor, you need add it in the device tree based on the SoC spec info.
To add Slave/client, you need add it in the device tree as subnode of master/adaptor node.