nds32: Make u-boot-nds32.h a private header

Rather than including this arch-specific header file in common.h, include
it from within nds32's u-boot.h header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/common.h b/include/common.h
index f36fed3..c1e6340 100644
--- a/include/common.h
+++ b/include/common.h
@@ -439,7 +439,7 @@
 #ifdef CONFIG_NDS32
 # include <asm/mach-types.h>
 # include <asm/setup.h>
-# include <asm/u-boot-nds32.h>
+# include <asm/u-boot.h>
 #endif /* CONFIG_NDS32 */
 #ifdef CONFIG_MIPS
 # include <asm/u-boot.h>