Update ad5766 spi engine

[PIoandan]

PR Description

The functionality of the ad5766 project was verified with SPI Engine. Over the old version it was created the ad5766_fmc.txt file for generating the system_constr.xdc file. Also it was updated the system_constr.xdc and Readme.md files. Tested with NO-OS. No correct signals were obtained from the DAC output channels.

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones

[laurent-19]

All FMC pins seem covered properly.

[laurent-19]

Should this PR include updating the SPIE hierarchy using the new SPIE script? Similar to cn0363 spi engine?
If not, the current changes seem fair to me.
Might be a good idea to state that the .txt contains a FMC pinout location description explicitly.

[PIoandan]

Should this PR include updating the SPIE hierarchy using the new SPIE script? Similar to cn0363 spi engine? If not, the current changes seem fair to me. Might be a good idea to state that the .txt contains a FMC pinout location description explicitly.

The SPI Engine does not support custom offload modules.