stm32mp: trusted boot support with FIP

STM32 board with a FIP image can have same trusted boot as STM32 image.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I2a7c4e4335af17cac7b74b0d6f1c65a74c60b22e
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184839


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>