Revert "mpc85xx: ddr: Always start DDR RAM in Self Refresh mode"

This reverts commit 2a5d5d27.
The commit breaks uboot boot (hang in ddr init)
on many PowerPC boards like P3041DS, P4080DS

Signed-off-by: Biwen Li <biwen.li@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>