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>
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index 81ccead..a3147fd 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -485,6 +485,14 @@
 };
 #endif
 
+#if defined(CONFIG_SYS_ARM_CACHE_WRITETHROUGH)
+#define DCACHE_DEFAULT_OPTION	DCACHE_WRITETHROUGH
+#elif defined(CONFIG_SYS_ARM_CACHE_WRITEALLOC)
+#define DCACHE_DEFAULT_OPTION	DCACHE_WRITEALLOC
+#elif defined(CONFIG_SYS_ARM_CACHE_WRITEBACK)
+#define DCACHE_DEFAULT_OPTION	DCACHE_WRITEBACK
+#endif
+
 /* Size of an MMU section */
 enum {
 #ifdef CONFIG_ARMV7_LPAE