warp7: Allocate specific region of memory to OPTEE

Subtracts CONFIG_OPTEE_TZDRAM_SIZE from the available DRAM size.

On WaRP7 we simply define the OPTEE region as from the maximum DRAM address
minus CONFIG_OPTEE_TZDRAM_SIZE bytes.

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

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>