ad4134_fmc: Fix constraints & update README

[StancaPop]

PR Description

The VADJ value for the ad4134_fmc is 1.8V (according to the IOVDD value). Fixed the constraints file to reflect the changes from the LVCMOS25 to LVCMOS18 IOSTANDARD. Also added the zed common xdc file contents and set the VADJ to 1.8V.

Updated the README files to contain de VADJ value.

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)
• Documentation

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