ppc4xx: Fix initialization of the SDRAM_CODT register

This fixes the initialization of the SDRAM_CODT register in the ppc4xx DDR2
initialization code. It also removes use of the SDRAM_CODT_FEEDBACK_RCV_SINGLE_END
and SDRAM_CODT_FEEDBACK_DRV_SINGLE_END #define's since they are reserved bits.

Signed-off-by: Carolyn Smith <carolyn.smith@tektronix.com>
Signed-off-by: Stefan Roese <sr@denx.de>