board: ea: mx7ulp_com: allocate specific region of memory to OP-TEE

On the iMX7ULP uCOM board, OP-TEE uses the memory region defined by the
maximum DRAM address minus CONFIG_OPTEE_TZDRAM_SIZE, so subtract
CONFIG_OPTEE_TZDRAM_SIZE from the available DRAM size to avoid
conflicts.

Note the OPTEE boot process itself subtracts the DRAM region it
lives in from the memory map passed to Linux.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Reviewed-by: Fabio Estevam <festevam@gmail.com>