stm32mp1: update defines to use it in firmware configuration

Add new defines and update the firmware configuration file
to use directly that defines. It will allow to directly
reflect BL32 size update during build.

Change-Id: Ieb11c9e0bac155d26fda9d0f3b086c55d5ce4783
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/186539


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>