arm: caches: manage phys_addr_t overflow in mmu_set_region_dcache_behaviour

arm: caches: manage phys_addr_t overflow in mmu_set_region_dcache_behaviour

Solved the overflow on phys_addr_t type for start + size in
mmu_set_region_dcache_behaviour() function.

This overflow is avoided by dividing start and end by 2 before addition,
and we only expecting that start and size are even.

This patch doesn't change the current function behavior if the
parameters (start or size) are not aligned on MMU_SECTION_SIZE.

For example, this overflow occurs on ARM32 with:
start = 0xC0000000 and size = 0x40000000
then start + size = 0x100000000 and end = 0x0.

For information the function behavior change with risk of regression,
if we just shift start and size before the addition.
Example with 2MB section size:
  MMU_SECTION_SIZE 0x200000 and MMU_SECTION_SHIFT = 21
  with start = 0x1000000, size = 0x1000000,
  - with the proposed patch, start = 0 and end = 0x1 as previously
  - with the more simple patch:
    end = (start >> MMU_SECTION_SHIFT) + (size >> MMU_SECTION_SHIFT)
    the value of end change:
    start >> 21 = 0, size >> 21 = 0 and end = 0x0 !!!

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>