arm: caches: Make DCACHE_DEFAULT_OPTION accessible for ARM64 arch

This fixes the following compilation error in ARM64:
arch/arm/mach-stm32mp/dram_init.c: In function ‘board_get_usable_ram_top’:
arch/arm/mach-stm32mp/dram_init.c:59:45: error: ‘DCACHE_DEFAULT_OPTION’ undeclared (first use in this function)
   59 |  mmu_set_region_dcache_behaviour(reg, size, DCACHE_DEFAULT_OPTION);
      |                                             ^~~~~~~~~~~~~~~~~~~~~

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