stm32mp: Add secure secret provisioning feature

This patch adds changes to support the secure secret
provisioning (SSP).

Add a specific platform build that constructs a dedicated BL2
image to support the SSP feature.
Supported boot mode is limited to serial boot (USB or UART).

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I85de08efd8d4183cd7e1bc0b6f17d247669b82ce
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/185336


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>