plat: imx8m: add a common dram PM code for imx8m soc

re-design the dram power management code to make it more
common for all i.MX8M SOCs. code need to refact and optimize
to make more better. Using this common code on i.MX8MM first,
for i.MX8MQ, will move to this later.

Signed-off-by: Bai Ping <>
(cherry picked from commit 8b2305c1464839bdf2683f8ece9482c52cd30720)
9 files changed