imx8ulp:ddr: saving the dram config timing data into sram

On i.MX8ULP, The dram config timing need to be saved into sram for
ddr retention when APD enter PD mode, so add this support on i.MX8ULP.

Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>