arm: caches: add DCACHE_DEFAULT_OPTION

Add the new flags DCACHE_DEFAULT_OPTION to define the default
option to use according the compilation flags
CONFIG_SYS_ARM_CACHE_*.

This new compilation flag allows to simplify dram_bank_mmu_setup()
and can be used as third parameter (option=dcache option to select)
of mmu_set_region_dcache_behaviour function.

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