imx8m: soc: Relocate u-boot to the top DDR in 4GB space

The EFI memory init uses gd->ram_top for conventional memory. In
current implementation, the ram_top is below optee address. This cause
grub failed to allocation memory for initrd.
The change updates DDR bank setup functions to place the u-boot at top
DDR in 4GB space.

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