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: