arm: imx8m: add OP-TEE node

arm: imx8m: add OP-TEE node

Add tee node in SoC u-boot device trees. Use a kconfig entry to specify
load and entry addresses for the op-tee image in the respective
defconfig.
Default IMX8M_OPTEE_LOAD_ADDR is supplied. To keep it simple, the same
addresses are used for each SoC as are defined in upstream tf-a
(BL32_BASE) [1].

[1] https://github.com/ARM-software/arm-trusted-firmware/tree/master/plat/imx/imx8m

Signed-off-by: Yannic Moog <y.moog@phytec.de>