stm32mp1: adapt context to manage monitor in DDR

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>