doc: stm32mp1: Improve OP-TEE related documentation
doc: stm32mp1: Improve OP-TEE related documentation

stm32mp15_optee_defconfig has been dropped from U-Boot as it became
identical to stm32mp15_trusted_defconfig.

Furthermore give a hint how OP-TEE is supposed to be installed.

Signed-off-by: Jan Kiszka <a5b45c9af271d077f30d36aa7d9b988eed9d56eb@siemens.com>
Change-Id: Id8f0bd84a87e3a62072dd4405aadddcdd3511213

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware