stm32mp: dt: fix issue when configuring the tzc400 region

TZC400 configuration is applied at the end of BL2 after MMU and data
cache being turned off. Configuration needs to retrieve the DDR size
to generate the correct region.
Access to the size fails because the value is still in
the data cache. Flushing the size is mandatory.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: Ib92c4c29f856638d479bb0600cebe2bb7de15312
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184733


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