Based off the Power-Up Initialization Sequence defined by the DDR2 Spec, CKE and ODT are supposed to be maintained at a LOW state while applying power.
To guarantee this occurs while the FPGA is being configured (before it can drive CKE and ODT "Low") we require the pull-down to GND.
Revision History
2/17/2011 - Initial Release
5/22/2014 - Updated DDR3 CKE requirement