feat(imx8m): add dram retention flow for imx8m family

Add the dram retention flow for i.MX8M SoC family.

Change-Id: Ifb8ba5b2f6f002133cf47c07fef73df29c51c890
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
6 files changed