The build process and images are on the wiki user guide.
AD-FMCOMMS1-EBZ User Guide [Analog Devices Wiki]
The way to add registers in HDL is simply modifying the up_[adc|dac]_[common|channel] modules.
As for accessing this from the Linux, one method is to prefix 0x8000_0000 to the register address.
You could also use the UI, for which it should list the device pcore.