Prepare section and coarse mmu mapping

Current mmu mapping is the "section" mapping. Section sizes are 1MB.
In this patch, "section" specific functions are moved from tee_mmu.c to
tee_mmu_section.c.

This patch prepares "coarse" mapping, that will be done using 64KB page size.

Signed-off-by: Pascal Brand <pascal.brand@st.com>