pico-imx7d: Reserve region of memory to OPTEE

Subtracts CONFIG_OPTEE_TZDRAM_SIZE from the available DRAM size so that
the OPTEE memory is not override during u-boot relocation.

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

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>