Add a new function to control the MCU subsystem
security state.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I070eec06fc93a1214227f25a6a4f1c40c66c86b0
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188430


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Gabriel FERNANDEZ <gabriel.fernandez@st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>