In case of monitor in DDR, BL2 exports the low power
functions using the context.
Adapt BL2 to embed the critical section and context
to share the corresponding information.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I198a50f3f15ab0e54d9e65e23da3d1c6fe3f8df5
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/182043


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