stm32mp1: rework MAX_PLAT_TABLES definition

Use a more flexible and easier to extend definition of
the MAX_PLAT_TABLES value.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I677cdfd7b403ad2bd2e8b7f6b0322ad74e81675f
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/185833


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