What is your host system?
The address that is passed to the ad5064_i2c_write() function is the register address and not the device address, so for channel 0 a value of 0 is OK. The I2C slave address is handled by the I2C framework internally and not directly referenced in the driver.
What is the configuration of the CA0 and CA1 pins on your device, the address of the device is only 0x20 if both those pins are floating. Also check that the power is properly applied to the device and that SDA and SCL are connected properly. If you have a logic analyzer please check if you can observe signals on the SDA and SCL lines.
Just to let you know that the problem was with the hardware. The AD5064 works as is with ltc2631-l10 on Variscite
DART-SD410 SOM (CPU: Qualcomm Snapdragon 410).
The only remaining question I have is regarding the warning message during bootup: 3-0020 supply vref not found, using dummy regulator. I use ltc2631-l10 in internal reference mode. Any advice on what that entry should be? I use this device tree: