nios2: Pseudo implement dcache_status/enable/disable()

AFAIK the NIOS2 architecture does not have any possibility to
dynamically switch off the data cache. So _status() always reports 1 and
_enable/_disable() just issue a flush of everything.

For example, common/cmd_elf.c depends on these symbols.

Signed-off-by: Joachim Foerster <joachim.foerster@missinglinkelectronics.com>
Signed-off-by: Thomas Chou <thomas@wytron.com.tw>

arch/nios2/cpu/cpu.c

@@ -24,6 +24,7 @@
 #include <common.h>
 #include <nios2.h>
 #include <nios2-io.h>
+#include <asm/cache.h>
 
 #if defined (CONFIG_SYS_NIOS_SYSID_BASE)
 extern void display_sysid (void);
@@ -47,3 +48,18 @@ int do_reset(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	nios2_callr(CONFIG_SYS_RESET_ADDR);
 	return 0;
 }
+
+int dcache_status(void)
+{
+	return 1;
+}
+
+void dcache_enable(void)
+{
+	flush_dcache(CONFIG_SYS_DCACHE_SIZE, CONFIG_SYS_DCACHELINE_SIZE);
+}
+
+void dcache_disable(void)
+{
+	flush_dcache(CONFIG_SYS_DCACHE_SIZE, CONFIG_SYS_DCACHELINE_SIZE);
+}