In v6.0 of the SPI-4.2, all SPI bus signals (RDat, RDClk, RDCtl, TDat, TDClk, TCtl) are set to LVDS without the internal device termination or DCI.
If you would like to use the internal device termination or DCI, you must specify this using the UCF constraint file. The correct UCF constraint is:
NET "RDat_P(0)" IOSTANDARD = LVDS_25_DCI;
Please refer to (Xilinx Answer 20017) for other supported I/O Standards for SPI4.2.