plat: imx: refact the dram retention flow on imx8mm

All the DRAM timing related config is saved by SPL in OCRAM_S,
so no need to do save for these configs in ATF anymore.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
(cherry picked from commit f0893b1f52303ba5604480db71efe605161666eb)
7 files changed