stm32mp1: mmu_set_region_dcache_behaviour

Since commit d877f8fd ("arm: provide a function for boards init
code to modify MMU virtual-physical map") the parameter of
mmu_set_region_dcache_behaviour need to be MMU_SECTION_SIZE
aligned.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>