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 <jan.kiszka@siemens.com>
Change-Id: Id8f0bd84a87e3a62072dd4405aadddcdd3511213