docs(st): set OP-TEE as default BL32

Recommend OP-TEE as the default BL32 for STMicroelectronics platforms.
SP_MIN is no more supported in STMicroelectronics software [1].
It will then no more receive new features, but should still remain
as it is in the TF-A code.

[1]: https://wiki.st.com/stm32mpu/wiki/STM32_MPU_OpenSTLinux_release_note_-_v5.0.0#TF-A

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ic49338dbba3fdcebcb1e477e6a1dbde32783482b
1 file changed