imx8mp_evk: Update LPDDR4 refresh time

Use more safer refresh time value for 6GB LPDDR4 on this EVK board.
Update the parameters for every frequency point.

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