As per (DS843) Table1 Notes9, 2 is the only value available for frequency ratio when the core is working in Dual/Quad modes in Legacy mode and Enhanced non-XIP mode, or in XIP mode.
However I can set any value in this field in the GUI.
What are the available settings for SCK_RATIO?
This is a known issue.
C_SCK_RATIO is fixed to 2 internally when the described operation mode is selected in Table 2.
This issue has been fixed in the latest Vivado version of the core.
The SCK_RATIO parameter is not selectable and fixed to 2 when these modes are used.