stm32mp1: fix TZC400 configuration against non-secure DDR

This change disables secure accesses to non-secure DDR which are useless.
TF-A already maps non-secure memory with non-secure permissions thanks
to the MMU.

This change also corrects some inline comments.

Change-Id: Id4c20c9ee5c95a666dae6b7446ed80baf2d53fb0
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>