Are there any timing constraints required for the MII to RMII shim core?
The only constraint that is needed is to constrain Ref_Clk to 50 MHz. The constraint should look as follows:
NET Ref_Clk TNM_NET = Ref_Clk;
TIMESPEC TS_Ref_Clk = PERIOD Ref_Clk 50 MHz HIGH 50 %;
This will be documented in a future version of the data sheet.