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>
diff --git a/arch/arm/include/asm/system.h b/arch/arm/include/asm/system.h
index 0eae857..43f7503 100644
--- a/arch/arm/include/asm/system.h
+++ b/arch/arm/include/asm/system.h
@@ -513,14 +513,6 @@
 };
 #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
@@ -578,6 +570,14 @@
 
 #endif /* CONFIG_ARM64 */
 
+#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
+
 #ifndef __ASSEMBLY__
 /**
  * save_boot_params() - Save boot parameters before starting reset sequence