verdin-imx8mm: adjust dram size in case bl32 is used
Adjust DRAM size in case BL32 secure payload is loaded (OP-TEE/Trusty),
so during MMU initialization U-Boot won't touch this mem area.
BL32 is loaded to the end of DRAM, bl32 payload size is read from
rom_pointer[1]. This relates to the issue described in
59efa6b52b("imx8m: Fix MMU table issue for OPTEE memory").
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>