Solution:
To slow down the edge rate (that is reduce drive strength), set the drive strengths from the default 8 mA to lower drive strengths using the Lattice Diamond ECO editor.
The ECO editor tool can be used to change the sysCONFIG pin settings.
You can access the ECO editor from
Lattice Diamond -> Tools -> ECO Editor.
Alternatively, the EPIC tool can be used to set the drive strengths and verify the settings as well.
