nios2: move nios2.h to arch asm directory

The nios2.h is nios2 cpu specific, and should go arch asm
directory.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/arch/nios2/cpu/cpu.c b/arch/nios2/cpu/cpu.c
index 36ea90b..39ae972 100644
--- a/arch/nios2/cpu/cpu.c
+++ b/arch/nios2/cpu/cpu.c
@@ -6,7 +6,7 @@
  */
 
 #include <common.h>
-#include <nios2.h>
+#include <asm/nios2.h>
 #include <asm/cache.h>
 
 DECLARE_GLOBAL_DATA_PTR;